This can be a frustrating problem that tends to happen when Revit has been installed after Navisworks, it leaves you without the Navisworks exporters or ability to use Navisworks Switchback with Revit.
This process shows you what to do with Navisworks 2013, the process is the same in earlier releases and with the standalone NWC exporters.
Firstly make sure the Navisworks Exporters are installed in Add Remove Programs, ensure these are the correct 32 or 64 bit exporters for your machine, if they are not there then you will need to install them.
Double click "Autodesk Navisworks 2013 32 or 64 bit Exporter Plug-ins", not the Language Packs.
Then select Add or Remove Features
Ensure Revit 2013 is checked, if it is not then check it but you may need to browse to the Revit folder.
Follow the Update process agreeing to any further dialogues.
Restart Revit, if exporters not available then restart machine.
You should now find the exporters available in the Add-Ins menu.
And in Revit 2013 you will also find the exporters under the Export menu.
If you still have problems please use the Navisworks forums to request more help.