I have nuendo but I havent use it for a really long time!
If i remember properly you need to load nuendo then start a new project and then open flstudio manually and it should automaticly rewire.
but like i said i only used nuendo like 5 times then statred using protools and i believe they pretty much work the same way
also i was cruising this site and i found a video that should work for you
http://69.50.201.40/download.php?id=4
thats for cubase sx but it should work all the same if im right as steignburg makes cubase and nuendo