Rewire starts automatically. Just open a MIDI track in Pro Tools, then open astereo aux or audio track (until PT 7 where you just open an Instrument track) then in the audio/aux track pull down a plugin menu and select Instrument. Reason should be there. As soon as you click Reason Rewire will start.
In the MIDI track select the output to go into the right channel in Reason (whatever the sound you are using) if you want to trigger notes from PT.
Hope that helps.