almost. You don't need to set the input of the music track to the bus, only the key input assignment in the plug-in itself. Then just hit the "key" button to tell the compressor to react to the signal coming into the key input. (edited after re-reading your post)
I actually wrote a blog article about this over at
the ProMedia Training blog a while back, but I'm too lazy to dig it up right now.
Have fun!