If you installed REAPER without the Portable option, ReaJS will look first inside Reaper's "Effects" folder in AppData, so you have to place it there, I believe, or uninstall REAPER/delete the AppData folder of Reaper. Of course, that is assuming you don't want to use REAPER in the first place (REAPER can load the JSFX by itself without ReaJS too).
Alternatively, you can have a look at reajs_info.txt and create a small reajs.ini and place it near reajs.dll with the path to the "JS" folder that contains the "Effects" folder (relative to reajs.dll), but unfortunately someone else said this did not work, but removing REAPER's AppData folder did work.
If that still doesn't work, sorry I honestly have no idea why, I think it would be best to search REAPER forums or ask Cockos then or post on REAPER forums about it.
Ok, I've added a simple octaved Supersaw example with the same (simple/slow moving) settings for the FFT Randomizer but with some phase randomization too (probably inaudible xD). The chord progression is slow because I wanted to highlight the effect, since it was set at 0.5Hz movement (on the only Layer), you could obviously make it faster but I wanted to illustrate same settings vs the white noise. I also used a slight compressor at the end of the chain, because this effect can get quite wild in dynamic range! In fact, it would benefit from more compression in my opinion for such "drastic" effects like this (42dB range for a single layer, the Magnitude Range setting), but I wanted to leave it more "raw" so people know what to expect, so didn't apply much compression.
Now one thing to keep in mind is that to hear the effect profoundly you need a "rich" input source, which is why I went with not just a supersaw but also octaved (it's doubled 1 octave below). This makes it able to work with more sound material, thus you can hear it much better, if that's what you're after. Of course I didn't play with the graphs at all so all frequencies have the same "shaping" randomization, you can see it clearly if you inspect the .mp3 with a spectrogram/sonogram, but you can change that, and per layer too.
Please note, however, that since it uses the FFT and not a Wavelet-based transform (not available in JSFX anyway), the shaping etc is linear, thus the majority of the effect will be in higher frequencies that most people consider "less important". This can be both a blessing and a curse. A curse because, well, most people care of the lower frequencies so they won't instantly find a use for this effect, but that's fine since I intended it to be more unique/experimental/not like anything out there, not to be super popular (although that's fine, but I doubt it will be hah :)). A blessing because there are incredibly few plugins out there that allow you to create dynamic shapes in the higher spectrum with enough precision. I mean, there's not much choice out there in the wild especially not free.
For instance, it can work great on "simple" input like a simple instrument, especially the phase randomization there, but in those cases it will be very subtle in effect but that's okay. It adds "dynamic variation" to otherwise static sounds. If you look at the spectrogram/sonogram of a real acoustic instrument you will see such "shapes" and variations in its noise/breathiness/etc AND in its harmonic/inharmonic tones.
It is my firm belief that this is something overlooked by a lot of developers out there! They focus too much on what every other synth/effect does, but looking at a synthesized sound versus a real sampled one is very recognizable purely by this "subtle" higher-frequency shaping and dynamic variations. This effect can help with that, too. A lot of things we hear are subconscious and not always immediately known "why" they give a certain vibe. I think part of the "magic" of real acoustic instruments is this subtle variation in not just the lower frequencies/phases but also the upper ones. It makes the instruments more "alive". For example think of a bowed or wind instrument and its 'breath' noise effect.
Most synthesized sounds use just filtered white noise. It's "static" and too simple (in shape). But real breath sound changes not just in amplitude, but in fact, its shape is constantly changing in time, subtly but subconsciously we "pick" this up, IMO. The human voice is incredibly complex in this "dynamic filtering". Synthesized voices ALWAYS lack this integral effect, you can inspect their spectrograms to see it yourself, that's why they always "sound the same" when you make them sing a long note or repeat a melody/passage. They have no "life" to them. That's why synthesized choirs work better than voices: this effect becomes less pronounced, because of the average random filtering of everyone in the choir, so it becomes more like "white noise" instead.
Of course with the FFT Randomizer you don't need to apply it just to the noise, you apply it to the whole thing! In those cases, "subtle" settings make the most sense. A range of 42dB in the shape is far from subtle (because it's both +/- so it's huge), for those cases you'd use 6-10dB per layer or less! And of course, phase randomizations too. Phases are mostly inaudible, but *changes* in phase add more dynamic variations (which is the point of the effect). Phase changes are noticeable in transients (it diffuses them) however. For example if you use it on samples, you'd get some sort of free subtle round-robins because no samples will be the same! They'll differ slightly in spectral shapes AND phases. Keep in mind for transients to mix this with the original if you do phase manipulations, otherwise you'll get a lot of diffuses transients, which can be cool but if you want to keep their punch I guess no. :)
I'm not saying that the FFT Randomizer does the exact natural dynamic filtering found in acoustic instruments. It doesn't use physical modelling at all (even though most physically modelled instruments, apart from stringed ones, lack this dynamic variation too...), but it's inspired from purely artificial "imaging software algorithms". If you used GIMP/Photoshop, you know of these "cloud" effects that are used to make artificial textures, it's the same principle. Natural things tend to be... ahem, "random". :)
Still, it helps to give some life into them, better than nothing.
Plus, you CAN use it experimentally and for sound FX design too. Like the two examples I posted, including this supersaw demo. These are exaggerated effects, but that's what demos should be for I suppose, so people get hooked to its possibilities. In these cases you use exaggerated settings to make the effect strongly known...
One last thing: if the linear frequency shaping bothers you and you'd like to use it logarithmically like other effects, you can always try to make the higher-frequency shapes more audible if you want, by pitching it down after, or use transformative spectral VST to reshape those higher frequencies down But for sound FX only, otherwise it throws the harmonics out of line. Still, such "dramatic" and strong effects are made more for sound design, anyway, not for the subtle variations I spoke of earlier.
Obviously, this is for sound design things, but I figured I'd say that the output from the FFT Randomizer can be *extremely complex* and varying as an input source to crazy modulated effects you can use after. So you can use it for *anything* to post-process it as a complex input. If you need a rich and dynamically varying "texture" for your sound design and want to process the hell out of it afterwards for effects, consider the FFT Randomizer at the beginning of the chain too. And you can guarantee that almost nobody will have the same texture, unlike if you use an already made texture from a sample pack :P.
Ok, this was a super long comment! I'll probably add it to its dedicated page, but thanks for making me realize it was lacking this kind of information. Being the developer of something makes you lose focus of what unfamiliar people will think of it. haha. :)
Oh and one more thing I forgot to mention, if you are bothered by the effect "carrying over" to the new chord for instance and you'd rather want it to be a completely different shape (good for transient effects or other fast attack cases), send it a Program Change a few milliseconds before the desired spectral change (and use the MIDI Force Change (ms) parameter to adjust how fast it changes).