To sidechain the Waves C1(sc) compressor, you need to have both signals going into the effects channel where the VST is insterted (so in Cubse you would have to use a Group channel). If you want to still be able to hear the sidechain signal, you'll have to use some sort of send arrangement, or thewre might be an option I've not used on the plugin....

Also, the sidechain input and the signal you want to be compressed must be panned to opposite sides (for example, kick panned hard (100%) left, bass panned hard right).
Then you just have to set the key mode on the C1 to L->R.
You'll also want to use some sort of centering plugin too, because without it, your bass/pad etc. will still only be coming out of the channel you originally panned it too.

Not much good if you wan't a sterio output though....