Rebumb - because I think I might be having the same problem. Working in the recently updated Cubase Artist 7.5.20. I activate the sidechain on the compressor on my instrument channel. When I try to send the signal of my kick (or any other audio source for that matter, I've tried different types of channels) via one of 8 'sends', the sidechain-compressor is simply not listed as an option. Strange thing however is that the sidechain-compressor IS listed as an option on the output bus (where normally it is set on 'stereo out'). Here it is working, but I don't think this is the right way to sidechain. Btw, any FX-channels I create DO show up as send options, and they work just fine.
I am a bit confused, am I doing something wrong or is this a bug? I haven't worked a lot with sidechaining, but for as far as I can remember I could always just simply use the sends option. It seems I have this probem since the 7.5.20 update (if it is a problem). I have tried different channels/instruments/sources and all appear the same. Restarting cubase did not solve the problem (also starting a new project did not).
edit: I think it's the same bug as mentioned in this topic (saw it only now): http://www.steinberg.net/forums/viewtopic.php?f=184&t=39640 / any solution?
There's no bug... Cubase works perfectly. I suggest you go on YouTube and learn how to sidechain properly... There are many ways to do it :D