I just put together the ArduPilot Mega (2560) board and IMU shield and using the APM Planner V1.0.93, was able to load the ArduPlane V2.251 firmware without a problem.
When I decided to upload the ArduPlane HIL V2.251 SIM firmware so that I can use the XPlane simulation, it fails. The progress bar shows the reading of the hex file and when that completes, the progress bar resets and it states loading 69728 bytes. Very quickly, the upload fails with an error message -
Check port settings or port in use? System.Exception: Upload failed. Lost syn. Try
at ArdupilotMega.GCSViews.Frimware.UplaodFlash(String filename, String board)
Without changing any settings, I can immediately upload the ArduPlane V2.251 firmware without a problem. It appears to be functioning properly as it returns flight data and does read the radio inputs.