Navisworks caches the dwg file to a nwc file and then uses only the nwc file. You must remove the nwc file.
Then Navisworks uses the prostructures.dbx to open the dwg and to cache it.
I have Navisworks 13 but I forced it to use the 18.2 dbx Loader. Dwg Loader Version is set to 2012.
Can be set with Global options.
AutoCAD 2013 needs a Service Pack 1.1 to load ProStructures Dwgs.
Same for the RealDwg 2013 as I can see with Bentley MicroStation SS3.
I guess a Service Pack 1.1 for NavisWorks 2013 will be offered by Autodesk later.
The AutoCAD object dbx entry for Pro_Stahl_App points to the Navisworks folder.
I have the AutoCAD objectenabler (objectdbx) key and the Navisworks objectdbx key set up.
Please look at my screenshots and try to reproduce.