Really simple
Hardpan your signals (100% left&right) and route into nitro.
Set op 1&2 to stereo width/balance, you should be able to hear op1 as it is routed to the output by default, hardpan this so you can hear your sidechain signal and hardpan op2 in the opposite direction. Make sure the output levels of op1&2 are turned right down (off). Route op2 to op3 and change the operator type of op3 to the desired effect (for the rest of this example I will assume you have set it to gain filter) and route op3 out (100%).
Click the 'env' tab and set the source to op1, on the 'mod' tab set env to modulate op3 gain/param 1 and set the amount to -100.

Hit play on your sequencer and make sure you have a signal present for both the sidechain and signal to be processed, you should hear the signal ducking when the sidechain signal is present (may have to change the sensitivity setting on the envelope), click on the env tab and play with the sensitivity, attack, decay and any other parameter that takes your fancy.

This is an especially good effect to use with the moog lp filter. Also try setting op3 to lp, op4 to a hp, routing op2 to op4 and modulating the level of op3. etc etc