I've released the FlightGear Mac OS X 1.0.0-r150. This release fixes the problem in installing additional aircraft/scenery data. It is recommended for all users to update 1.0.0 (r149) to 1.0.0-r150. You can check your revision number by clicking "About FlightGear Mac OS X" from the menu bar of the GUI launcher.
If you don't want to download entire FlightGear package, you can also get a single fixed file and copy it to FlightGear folder yourself. see detail instruction at this page