NWNavigator only worked on 32-bit AutoCAD (and vertical) versions.
Navis Manage 2011 now allows NW Navigator to work on a 64-bit AutoCAD. You don't even need to register Navis 2011, just install it for it to work.
Note, that if you still have an older Navis version loaded, you'll be prompted for which version to use when you make an NWD. That preferance will be set for the entire AutoCAD session (you'll be prompted the next time you start AutoCAD).
If you refresh the NWNavigator browser first, before creating an NWD, it'll automatically default to a 2011 Navis file format so if you need to sent navis files with others using 2010 navis, export a NWD to the 2010 format first, then you can start using the Navigator palette.