In Pro Tools, instantiate the Rewire application RTAS plug-in on the insert of an Aux Input track - this should automatically launch your Rewire application (Reason, Live, etc.). In the Rewire plug-in window you will see the name of the Rewired application - next to the name is a drop down menu that will allow you to select the channel from the Rewired application that you want routed to the Pro Tools track.
Please note: Only the first 2 channels from Reason can be routed to a stereo track in Pro Tools. For information on how to route Reasons mono outputs to a stereo track in Pro Tools, please see (Digidesign) Answerbase #27105.
Propellerheads also has an excellent guide available at: www.propellerheads.se