first Open Nuendo then open reason , and the trick is done...to make sure it is rewired look on the hardware intefrace of reason once u expand it u should see on the display Rewire slave..If you really proficient in batch programming you can create a batch file that will open a default nuendo project and then open a default reason project of your choice.
Once both programs are running go to Nuendo and open devices-->Reason and turn on all the channels that u r using in your project. Usually if reason is open with the default project only the signal from the mixer is routed to the hardware interface so you will just have to turn Mix L and Mix R in the reason rewire interface in Nuendo. Another option is to bypass reason mixer overall and route the reason devices manually to the hardware interface, but please before you do so name all your devices correctly, believe me that will save you time and future headaches.
For recording there's another post on this forum about that, i try to answer that person's question so refer to that post
