Oh man, this was moved into 'Getting started', making me look like a jerk.
Firstly, check out
www.tweakheadz.com and look around for their introduction to MIDI and digital audio. If you're thinking of plugging MIDI cables into your mixing board, then I think you need to learn the difference between MIDI and audio; if you don't, then there's not much chance you'll be able to get things working properly. Once you've looked over that stuff and feel comfortable with it, try the following.
1) You've skipped the tweakheadz page, haven't you? OK, well don't blame me if you don't understand what comes next. Alternatively, actually do some work and read
http://www.tweakheadz.com/how_to_get_st ... _midi.html and
http://www.tweakheadz.com/setting_up_yo ... o_card.htm.
2) I will assume that you have a MIDI interface for your computer that is running Reason. Take a MIDI cable, and connect one of the MPC's MIDI outs to the MIDI in of your computer's MIDI interface. This will enable the MPC's sequencer to trigger Reason.
3) If you want to hear Reason and the MPC at the same time, which I would think you do, then the simplest way is to use a mixer. Hook up the audio outputs of the MPC to the mixer's inputs, and the audio outputs of your PC to the audio inputs of the mixer. Connect the mixer's output to your amp/speakers/headphones/whatever, and away you go.
CAN SOMEONE WITH A LITTLE COMMON SENSE SHOW THE PEOPLE IN THE FORUM HOW TO CONNECT MIDI CABLES AND ALL TO THE MPC AND GET THE SOUNDS TO SYNC TOGETHER. SHOW SOME KIND OF VIDEO ON HOW TO STEP BY STEP CONNECT MIDI IN TO MIDI OUT TO MIXING BOARD OR WHATEVER. PLEASE DON'T SHOW HOW TO SYNC REASON TO THE MPC SHOW HOW TO CONNECT THE BACK CABLES TO THE MACHINE. 
