Hi Dai, I see this in many missions and have even made the mistake myself. It's easy to forget about it.
Before uploading your mission you can edit your FLT file to get rid of that nasty no flight plan warning that everyone else gets except you.
In your FLT file, under the [GPS_Engine] section, an FSXME made mission will include the file name path to your Public folder when it created the mission. It is like this so it can create an installer, if you wish too, although most folks hate them. That is why ME puts your new mission in your My Missions folder in your FSX root folder(so you can fly it) and also one in your Public User folder under the Mission Wizard# folder(so ME can do it's magic).
It looks like this(from one of my missions)
[GPS_Engine]
Filename=C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\Missions\My Missions\WizardMission5\Triple Tree FlyIn S.C.Plan
All you need to do is get rid of the whole path except for the actual PLN file name. After the edit, when a download user flies the mission, FSX will now just look in the mission folder for your flight plan(PLN).
When edited it will now look like this:
[GPS_Engine]
Filename=Triple Tree FlyIn S.C.Plan
If you do not want to include a flight plan in the mission folder, although your original mission was made with one, leave the file name blank but keep the GPS section in the FLT folder. This will happen if you make a mission in FSXME with a flight plan for build reasons but chose not to include the plan to coincide with your mission scenario.
Hope this helped, I tend to ramble
BTW The sheik commented that his camel could fly a heli better than me.