Are you sure it's the AIR file?
If the AIR file came from an AI aircraft it's coded to not show up on the menu. The fix is to open the AIR file in AirEd and change the aircraft type from 2 to 1. Unless you can't open it in AirEd. That would be odd; I've never heard of such a thing. (Could it be a Windows thing, maybe a ,mater version of Windows preventing you from doing something that you normally ought to be able to do???) You should know if the file came from an AI plane. If it didn't, then...
As far as I know, the only other way the AIR file would prevent an aircraft from showing up in the Select Aircraft menu is if the file name of the AIR file doesn't match the name listed in the sim= line of the aircraft.cfg file. So maybe you have a type in that line of the cfg file? Note that if the typo is in the UI section for the first variant on the list, that will prevent all of the following variants from showing up. If this is the problem, edit the UI sections of the cfg file to match the file name of the AIR file. In that case there will be no need to edit the AIR file.