Rewire is a brilliant tool. If you use rewire to run reason through your sequencer(ie cubase, logic), you can open Massive as a vst and use reason and massive at the same time. also this means you can also use any other vst instruments and plugins at the same time. It's a little bit more effort than the other ideas however it makes such a difference and has a lot more possibilities. Hope this helps blud
open reaper, add a new channel, right click the channel and 'add a virtual instrument to channel' in the window that pops up click vsti's or dxi and open massive.
then do the same, but instead of clicking the vsti or dxi tab, go to rewire, and chose reason. reason should open automatically in the background, and use the three softwares together easily.
just make sure in reaper you chose the bpm of the track your making, otherwise it makes reason and massive sync to 120bpm (reapers default bpm, you can change this just below the horizontal midi timeline scrollbar).