Aye yo check this out. First you have to have a midi-to-usb interface to hook the MPC to the computer. After you have the mpc hooked up to the computer, open Reason(no "S" on the end of the name) and set Reason to recognize your midi interface. After you have done that all you have to do is set the midi channel on your mpc to to the same midi channel in Reason.
ex: MPC Drum track 1
MPC Midi 1a track 2
Reason: subtractor---channel1 on the midi bay in reason
Now the MPC should be the main controller of Reason ( no "S" on the end of the name)