Let me preface my answer by saying I haven't used Guitar Rig, and I don't use Cubase. I THINK I can answer your question, though.
The ReWire standard provides an interface between two sequencers (like Reason with Cubase) - that has nothing to do with the this situation. You're interested in another standard - "VST" - which is used by sequencers (like Cubase) to host effects (like Guitar Rig) and software synthesizers. Guitar Rig DOES have VST support (as well as support for a lot of other standards, but since you're using Cubase, you just care about VST). So, if you've already saved your amp model...
Open Cubase, and create an audio track for your guitar part. Now, you want to insert Guitar Rig as a VST effect on that track - your Cubase manual should be able to walk you through that. You may have to register Guitar Rig as an effect within Cubase - if you installed it to the default directory, its probably already accesible, though. Once Guitar Rig is inserted, you should be able to open the instance of it that you've created there, and load the preset you already made for it. What I don't know, is if you'll be able to hear the effect while you record, or if you'll have to record clean, and only hear your sim-amp during playback.
If you're still having problems, you might have more luck in the Steinberg/Cubase forum.
