In your scenery library, are the files for the MegaCity Dallas above the FSDT KDFW file? If so, you must move the Megacity files down the list until all files are below the KDFW file. If the Megacity files are above KDFW in the list, this means they are in higher priority than the FSDT airport, which means that they will come up on top of the airport, which is not what you want. So move all the files down the list until they are all a lower priority level than KDFW airport. This is a very common error with Megascenery/MegaCity and airports that I have had, and I fixed it by moving the airport to a higher priority level. Hope this helps
