Originally Posted by INGRAMC
Are you using the 32 or 64 bit version of Reason? I don't know much about cubase but if version 5 is a 32 bit app and you have a 64 bit version of Reason that will cause it not to show up. If that is the case you will need to do a seperate 32 bit Reason install and this will resolve the problem. I had the same problem rewire via Protools.
The OP is using Reason 4, which is 32bit only.
I'd double check Cubase is in 32bit mode if I were you. Other than that, I don't see anything that could be the problem.