We currently offer a massive range of downloads for FSX, as well as older flight simulator add-ons for the ever-popular FS2004, FS2002, CFS3 and now becoming ever popular with dediced virtual aviators is Laminar Research's X-Plane series.
You need the FSX Deluxe version. Install the Software Development Kit (SDK). There are instructions in there on how to install. You want to open and read the fsxsdk.chm file. There is a section on Mission Development.
Here is a good link for additional information on Missions: