From X-Plane SDK
XPLM_API void XPLMSetFMSEntryInfo( int inIndex, XPLMNavRef inRef, int inAltitude);This routine changes an entry in the FMS to have the destination navaid passed in and the altitude specified. Use this only for airports, fixes, and radio-beacon navaids. Currently of radio beacons, the FMS can only support VORs and NDBs. Use the routines below to clear or fly to a lat/lon.