Hi mate, I had a quick look for you but theres no sidechain input in retrosynth, so heres how to get it to pump to the kick from the ultrabeat (trance gate)...
this technique will apply to any audio track or instrument you want to "duck" or "gate" for the matter....
on the "retrosynth" channel strip add/open the "noise gate" plugin (dynamics > noise gate" , then in the top corner of that plugin you see sidechain now.... selct the ultrabeat track or channel from there thats it. the default logic gate should have it pumping. NOW adjust the gate to what ever your pumping needs etc...
cheers & have fun...