Q: how did you check for updates?
Try the following:
1) Go to the Apps Page. The page in your library that shows all apps you have.
2) At the top right click on 'Check for Updates'. It'll be a blue text link.
3) Wait 1-2 minutes while it checks.
4) You'll get a dialog that shows you available updates. Click on Update next to Netflix.
5) Restart your Nook.
Power off: Press and hold power button for 20 seconds.
Power On: Press and hold power button for 4 seconds.
Also, if by any chance you went to settings and tried to register netflix through there, don't do that. You should log in only via the NEtflix app itself.