Download the plug-in for the .stl converter (url below), click the folder for the version of Revit being used.
Restart Revit and an Add-ins tab should appear on the menu bar.
Click External Tools, a pull-down menu should appear, and then click STL Export. Save it as binary and the Categories should all be grey and checked. Click Save.
If the stl export did not work or created strange surfaces, then load your file into a new project and repeat the above steps.