either record the MPC's audio output into your computer, see
http://tweakheadz.com/setting_up_your_audio.htmor resample your finished tracks, which creates a WAV file that you can transfer to your computer (via USB or CF card). You can convert the wav to mp3 on the computer if that's what you want. search the manual and the forums for sampling from the main outs.
this has been covered hundreds of times in this forum