basically what
synthsonix said in logic if your using a software instrument (ultrabeat/exs24 sampler ect) set the output to a bus then on the bus channel in the mixer set it to "no output"
If its audio just set it to no output then you just have a signal. (Dont mute anything because then there will be no signal to side chain)
if you want to check your getting the side chain signal open a compressor and set it to side chain if you get stuck then you can see the signal easier.
cheers Jamie