If the 303 is anything like the 505 (and I think that it is), you should just run the outputs of your MPC to the ins on the 303, then the outs on the 303 to your mixer/soundcard/amp/monitors or what have you. On the sp505 there was a way to monitor the audio coming in on the inputs, I thing it was a button labeled "ext", the 303 should have something similar. Once you hear the audio from your MPC, just apply whatever effect you want.