Both the ReWire host and client apps need to be running in the same bit mode for ReWire to work. If your host application is 32-bit, you need to run Reason in 32-bit mode as well. This can only be done in Reason 8.1 and earlier, since Reason 8.2 and later are 64-bit only.
If you have a Reason 8 license you can download Reason 8.1 here. If you are using a Mac, you change the bit mode by right-clicking the application icon in Finder and selecting "Get Info" and toggle the "Open in 32-bit mode".
Tip! To make sure both applications run in the same bit mode on your Mac, launch the Mac OS "Activity Monitor" utility and check the "Kind" column for the Reason and host applications.