Hello, Im new with FSNav myself (just bought it 2 days ago), but I think with FSNav, you have to export your flight plan into your flight simulator flight plan folder and then load it up in FS. 

With sids and stars, ive always flown them without fsnavigator.  Just print out (or put em on your kneeboard) the charts, and read the instructions pretty much.  If your finding a waypoint on the STAR or SID, just hit the direct key on your gps and enter the identifier on the GPS, and after each waypoint, enter another one.  If you have a flight management computer such as the one on the PMDG aircraft, then even better!  Just enter those waypoints onto your flight plan!