From it's homepage:
To configure DualComp as a sidechain compressor, add it to the desired channel and then create a new channel with the signal for the sidechain input (usually a kick drum or something similar). Route this channel's output to ports 3/4 of DualComp on the primary channel. The sidechain signal will not be added to the output of the primary signal, so you should create a separate track for your kicks.
Sample configuration of DualComp in Ableton Live:
