Reaper doesn't come up as a Rewire option in the mixer window under inserts on ProtoolsLE v8.03 either. I'm on a Macbook Pro core 2 duo OSX 10.5.8.
Perhaps this is better reported on the Issue Tracker here:
http://forum.cockos.com/project.php?issueid=2152
Although it's not exactly the same issue.
Perhaps the moderator can move this thread onto either the OSX forum or the Pre-Release as is appropriate.
Note: I'm running Reaper v3.36pre4