1) Plug Midi out USB male midi plug into MIDI Out Port A on the back of the MPC
2) turn on MPC then Turn on Reason
3) go to MIDI/Sync (MODE+PAD 9)
4) Go to Sync tab (F2)
5) click over to Sync out and turn off to A (Master)
6) Go down to Mode (on screen, not the button) and make sure it is on MIDI CLOCK, also turn SEND MMC to ON
7) Go to Reason and go to preferences

Go to page and click on it and go to MIDI and choose your USB to MIDI interface
9) then go to Advance MIDI and go to BUS A and choose your USB to Interface again and also go down to the Misc area and choose your USB interface again in both Remote Control and MIDI Clock Sync (not sure if you have to do this, but I do it and it works for me so I always do it, it wont hurt anything, it only helps)
10) Now go to the Rack area of Reason and scroll to the top of the page and go to your MIDI In Device and now on each Channel on the MIDI In Device can be applied to each module in Reason (click on the arrow thing on each channel and choose the module you want to be controlled by that MIDI channel)
11) now go to a track on your MPC and click over to your MIDI option area on the main screen and choose a MIDI channel and control the corresponding channel in Reason with that MPC track.
p.s. make sure you turn off your program on your MPC track
hope this helps
"I find this very stimulating to the mind, but drives me crazy at the same time,..."
- K-OS