Hold the SHIFT key down when you select Options from the Application menu to access additional Registry settings. Some good stuff there. Note: This does not work using the F12 shortcut key.

Creating a custom avatar that is meant to look like a piece of heavy equipment like a forklift or front end loader is best done by exporting a Revit family to Navisworks using the exporter. The family should have the back facing towards the top. I have attached a family for reference.

The process is take your model in Revit and export to Navisworks to get your NWC.

In Navisworks open the merge the NWC into a new file and save it as an NWD to the folder below substituting the name you want for the avatar where <avatar name> comes up.
C:\Program Files\Autodesk\Navisworks <flavor> <year>\avatars\<avatar name>

Close out of Navisworks and restart it.

Once restarted in a project you can go to the 'Viewpoint' tab and on the 'Save, Load & Playback' panel click 'Edit Viewpoint'. the 'Edit Viewpoint' tool is found on the middle right of that panel.

This will open a new dialog box where you want to click on 'Settings...' under the collision button.

Here you can enable your third person if it not currently enabled and choose an avatar from the drop down that you just created. Some settings will have to be worked with in here such as the distance and height to get the avatar to appear correctly. Some scaling may also need to be done in the object transform settings in the NWC itself as well to correct the sizing.