I deleted my earlier post. The language I used was unnecessarily harsh. I'm sure the SoundMagic dudes are doing the best they can. I just wish their "best" was a little better. Truth be told, I'm still irritated that they don't make a demo version of their China Story Erhu for Mac. I've bought too many instruments that turned out to be dogs to spend the bux on yet another one without at least being able to try it.
By the way, your question doesn't sound stupid. The instructions explicitly say to run the SPRUNTIME.pkg. As it turns out, apparently that was necessary on the Mac for older versions, but it's not required for the current version. If you just drag the plug in into your /Library/Audio/Plug-Ins and then Components folder (for AU) or VST folder it should work OK.
Nevertheless, the new (v1.2) Orchestral Strings instrument is not one I'll be using. After I installed it, I found that it's not polyphonic. That's a major FAIL. Fortunately, I kept the ZIP file for the previous version, so I've reverted to that. It's not the most natural sounding strings instrument, the it works in certain applications.
Oh...and you can't tell which version you've got by inspecting the plug-in in the Finder, because the two most recent versions both give the version number as 2.2.1. You have go by the "Date Created". The previous (polyphonic) version was created on 2012, November 30. The current (monophonic) version was created on 2013, January 18.
As far as their pianos go, the only one I've tried is the free Piano One. I guess it was OK, but I wasn't blown away by it. But then, I'm fussy about pianos. I've lost count of all the piano instruments I've tried (including some very expensive, humongous, deep-sampled libraries). They all have their virtues, and even the most expensive ones---which sound fabulous when you play them solo---can suck dead hog's eggs in a mix.
And there's no consistency, either. Whether any given piano sounds good in a mix is totally dependent on the sonic character of the mix itself. A piano that sounds great in one mix will sound like dog crap in a different mix. In my experience, there's no single piano instrument that is absolutely going to work everywhere. So far, the only strategy that works (for me) is to have several different piano instruments and choose whichever one works best for any given mix.