Hmm, I'm not sure whether Cubase 3.7 can do this. I've only ever tried Cubase since v5.0. But I can tell you how it's done in Cubase 5 at least, maybe this will work in 3.7...
Load Rez Synth somewhere. It will of course be processing an audio track or tracks, but what's maybe confusing is that next you go to a MIDI track & then assign the output of the MIDI track to Rez Synth. Rez Synth needs to be turned out (i.e. not in bypass) for it to show up in the MIDI output devices menu. Once you've chosen it as the output, then that MIDI track will send all recorded & live MIDI to Rez Synth. I made a picture for you: |