there are 2 ways that max can use rewire: 1. as a slave, by setting the audio driver to ad_rewire and using objects like hostcontrol~ and hostsync~ 2. as a master, by using the rewire~ object

since reason only functions as a slave, you'll want to go with the second, hosting reason "inside" max.

i should warn you that i've heard a lot of people complaining about oddities when using the rewire~ object...