Hi DavidMettler1819,
You can delete the following registry keys that the installer uses to determine whether the “Autodesk Navisworks 2012 64 bit exporter Plug-ins” are already installed:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\8E80B0CB49149040C8FEFB957908E161
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\8E80B0CB4914CFD4C8FEFB957908E161
Warning! Problems caused by improperly editing the Windows Registry could render your computer operating system unusable. Microsoft provides a wealth of critical information that you need to know about the registry in the Microsoft Knowledge Base.
Use the Microsoft Registry Editor only at your own risk and only after backing up the registry as well as the system.dat and user.dat files as outlined for your operating system in the Microsoft Knowledge Base and in the related Autodesk solution, http://usa.autodesk.com/getdoc/id=TS66513. Additional information about the registry is also contained in the Help topics in the Microsoft Registry Editor.

There will be two keys listed. One is for the exporter and one is for the exporter’s language pack (stated in the “ProductName” key). Delete both the 8E80B0CB49149040C8FEFB957908E161 and 8E80B0CB4914CFD4C8FEFB957908E161 keys. After doing so, when you run the installer, it should no longer state the exporters as being already installed, and you can then reinstall the exporters over themselves. Hopefully this way will correct the issue with the add-in in Revit as well, but it should add the language pack back if you want to uninstall the exporters.
Best regards,
John Lipp
Support Specialist
Product Support
Autodesk, Inc.