STudio one , fx buss not available as input

[gentleclockdivider]

OK, now try these extra steps:
Step 7: Create another Bus channel (Bus 2)
Step 8: Route the destination of FX 1 to Bus 2.
Step 9: Click on the input dropdown of the audio and select Bus 2

This may not be the tidiest solution possible, but it works to achieve what you want, nonetheless.

I am verry well aware of that , read the fourth post

[jamcat]

Well it looks like the best solution is to upgrade to Studio One 6.