You can see how to work with the rewire~ object and Reason in this article.
The rewire~ object cause Max to act as a ReWire "Mixer" - a host for other ReWire "Client" applications. As a result, you need to launch a Max patch containing a rewire~ object before you launch the ReWire applications. Once you do, any subsequent ReWire Client application will see the availability of a Mixer, and will automatically attach to that application (rather than starting in standalone mode). You also have to quit Client applications before quitting Max so that the connection between applications can be properly removed.
An application cannot be both a Mixer and a Client simultaneously. Thus, you cannot use the rewire~ object within a patch if you are using the ReWire audio driver to connect to another Mixer application.
You can find out more about ReWire on the Propellerheads' web page dedicated to the technology: here