Just started doing that about a month ago. It's great. Reasons instruments are much better than Abletons. No contest.
Not too sure what you're asking, but here's how it's done. Open Ableton, have an audio track, and midi track out. For audio switch "auto" to "in", then change the "audio from" to "reason".
Next open reason. Once reason is open change the midi tracks "midi to", to "reason".
Open an instrument in reason, and choose what you opened in the under "reason" in the "midi to" section of your midi track in Ableton.
I really have no idea how much you know about reason and ableton, so I guess I wont go any further. You might already know all of that, or I might have to tell you some more as well..
There's also probably a youtube tutorial, which is probably better cause I imagine I explained that really vaguely, and made it sound a lot more confusing then it has to be.
Yeah I assumed there was a youtube tutorial, but that actually helps a lot.
I know a good bit about Reason, but none about Ableton really except for the fact that i CAN rewire.
I'm too lazy, stubborn to try it lol