Both Cubase and Reason need to be using the same option, either both 32 bit or both 64 bit. I don’t think Reason 5 was 64 bit, so I’d suggest loading C7 in 32 bit (it will not affect your 64 bit version).
Start C7, Start Reason. Open a new empty project in Cubase, open a new empty project in Reason. In Cubase, go to Devices, select Reason Rewire, and activate the desired channels. I hope this information helps!