Well, after now having used FSX for over a week (!), I thought I'd see what this Mission Builder thing is all about.
I had no success at all.
Even after installing the SDK, reading the guff about the dll, editing the dll.xml file correctly, checking it's proper path and location, going to the above fsxmission site and double-checking that I'd followed the proper procedures........it doesn't appear in FSX.
Nothing that Microsoft creates as an asistance for any opened-ended progamming is easy to understand, even though you pay good bucks for the extended product. Take for instance, FS2002, without the genious of the fellows who are script wizards, and flyers, we would have never figured out how to use the LWMs, and VTPs without their tools to help move the community into production of FS2002 scenery. What a mess that was to understand in the SDK.