Tip

Store FMS files into the Output/FMS plans directory inside the X-Plane directory if you would like to use the flight plan in the X-Plane GPS, the X1000 or the FMS.

You have to load the FMS flight plans from the stock FMS or GPS when sitting in the cockpit and avionics switched on. Do not try to it as a .sit, .rep or other file type from the X-Plane menu.

See here in the user manuals for the respective units: X-Plane User Manuals Look for a chapter Loading a (saved) Flight Plan in the PDF manual for the navigation device.