Perhaps this may be of some assistance...still learning myself.
A little known trick, but all X-Plane aircrafts have an FMC (visible or not) into which your can upload an FMS plan.
For example the Cessna 172 has a GPS430 with very primitive capabilities, but in the back ground there is an FMC that can direct the GPS430.
Steps to do:
•Define a key referring to FMS/key_load (I use the backslash key "\").
•Create the flight plan and save it as .fms file.
•Copy the .fms file to C:\X-Plane 10\Output\FMS plans\ (or wherever you have your X-Plane installed).
•Start X-Plane and Cessna 172 and locate the plane on the starting airport of your flight plan.
•Type "\" (without the quotes) and select the flight plan and load.
•First waypoint will show on the GPS430.
•Take off with runway heading set until 400 to 1,000 ft AGL.
•Switch CDI to GPS mode, activate LOC and activate the autopilot to fly the course.