i downloaded some new aircraft and unzipped them. Then I moved the files one airplane at a time to the aircraft section of flight simulator's folders. Then when I try to run the new aircraft in the game they are nowhere to be found.
They may be listed under the manufactures name in the aircraft main menu.
Put "ANY" in the top window, then look in the center window for the aircraft.
Try not to load more then one at a time, if you have problems it's easier to tell which it is if only one has been added.
If they are textures, or one that is from 2002 or still in a "wrapper" it may not show up.