It's like when you use a VST instrument in Cubase. When you have a midi track opened up, you can assign where you want the MIDI data to go.
Make sure you open reason right after you open cubase, and this has to be done before you try to assign the MIDI outs. Once they're both open, pull up the assignable outs for that channel in Cubase. It should show all the rack modules in reason along with whatever VSTs you have opened. From there you can play MIDI data in cubase and it should respond in reason.
Now also, once you've rewired reason to cubase, you have to make sure you set each rack sound to an individual out on the top of the Audio out box in reason (unless you want all your audio on the 2 outs that they default to). Make sure you have an audio track set up in cubase so you can hear the sound from reason. I hope that helps.