Reason and the MPC both have a MIDI "Beat Clock" at the core, an internal electronic pulse that dictates the speed and rhythm of all the sounds generated. In order to get both sequencers to work in perfect parallel, you instruct each to share one Beat Clock. For this to happen, one device must be designated the "Master" and the other the "Slave."
Most MIDI hardware instruments are capable of outputting a Beat Clock signal, as well as receiving one from an external source. As "Reason"'s Beat Clock is generated by the PC or Mac's internal clock, it is not capable of outputting to an external source. As a result, the MPC must be designated as the Master in order to sync the two devices.
Once the MPC is connected to the computer's MIDI interface, open the Reason preferences dialog. On a Mac, this involves opening the "Reason" menu and clicking "Preferences." On a PC, "Preferences" can be found in the "Edit" menu. Select "Advanced Control" from the drop-down box at the top of the menu. In the "MIDI Clock Sync" field, select the MIDI input port to which the MPC is connected. There will be a green check mark next to the port. Disconnect any devices bar the MPC if there more is than one check mark. Close the "Preferences" window and click the "Options" menu on the top toolbar. Navigate to the "Sync" sub menu and select "MIDI Clock."
Open the MIDI/Sync menu on the MPC by entering the "Settings" dialog. Hold down the "Mode" key and strike sample pad number seven to enter the menu. Turn the value dial until it displays "Sync." Scroll down to the line below using the cursor arrow keys to enter the "Sync Mode" field. Turn the Value dial until the Sync Mode displays as "Master." Press play on the MPC and the Reason sequencer should play in synchronization.