Hi there,
If you're running Navisworks Manage 2010 32-bit, you will need the 32-bit Civil 3D 2010 Object Enabler installed against Navisworks, and set the DWG Global Option: Loader Version to 2010.
If you're running Navisworks Manage 2010 64-bit, then I'm afraid you're out of luck. At time of writing there is no Civil 3D 2010 Object Enabler for 64-bit.
I hope that helps,
Jeff
Senior Software Engineer
Navisworks
Autodesk Ltd.