On 64-bit Navisworks, there is no support for AutoCAD 2007 Object Enablers as AutoCAD only added 64-bit support in AutoCAD 2008. 32-bit AutoCAD & RealDWG are not supported on 64-bit operating systems, and thus Navisworks only offers 64-bit DWG handling.
The alternatives available are to use the 64-bit 2008 (or higher) Object Enabler for the relevant DWG file, or to export on the 32-bit CAD application & machine to NWC & load the exported NWC on the 64-bit machine. The DWG file format itself is 32/64-bit agnostic so as long as you can use the relevant Object Enabler, compatibility should remain. The ADT/ACA OE is included as part of the Navisworks installation, so the file should load correctly without additional installation.