To create an FS9 flatten file I normally use SceneGenX, but that's probably overkill for just one airport. You could use FSTFlatten instead, which is probably easier to learn.
I've created a quick flatten file, if you want to try it.
Use the Change Airport Altitude function in the ADE Tools menu to change it to 3100 ft. After that and compiling the airport it looks fine.
Hope this helps,
ADE did place a MT84_ADE9_BL_ALT.BGL in the ..scenery/world/scenery folder, which is paired with the MT84_ADE9_BL.BGL file I place in the ..\addon scenery\MT84\scenery folder I created.
The problem I encountered was that the actual airport was then
inside the mesa, because I couldn't seem to create a "flatten" that would work.
I discovered that trying to use a flatten.0=xxxx in the scenery.cfg does not seem to work (or at least I never managed to flatten more than the NE corner of the mesa, despite having created a flatten rectangle that was more than 4x what was actually needed.
I found what was undoubtedly a neat tool at one time called "FlattEx.exe" that can grab the coordinates directly from FS9 and create a proper flatten.n=xxx,xxx etc. line for use in the scenery.cfg file. I also learned that one should never, ever alter the template copy of scenery.cfg in FS9's root folder if one is running Vista...
A google search revealed that a fellow had created a set of files specifically for MT84 (cleverly named mt84fix.zip), which while indeed flattening the mesa, left behind an oddly shaped grassy rwny, but with a wierd, offset dotted white line that extended past both ends of the 2,000'x25' strip.
I also had a chance to look at SBuilderFS9, which while undoubtedly a wonderful bit of programming, is
far too complicated just to learn how to accomplish such a pedestrian goal as mine... a simple flatten!
In any event, it's been a terrific learning experience. Thanks for the "flatten," Tom. With that and ADE I've managed to accomplish my modest goal(s). I changed the surface to "Bituminous" and added a small bituminous apron and a three bay hanger. Here is the result: