Save the flight to Flight Sim X format (Documents\Flight Simulator X files). Load the flight with the 707 AFTER the 707 flight has already been started, because sometimes the default dpl.PLN (needed for the DNS to work) remains as the flight plan if I load the plan before I have started the flight from Free Flight. Connect FSC 8.6 GPS to FSX (I hide the GPS window). The 707 will follow the flight plan, you will hear the sounds as the 707 passes the waypoints in the plan. I have found that the FSC X (8.6) and FSC 9 GPS does not work on all payware aircraft. As far as I can remember, it does work on all the default FSX aircraft that have an autopilot.

Hope this helps.

Mark