I've fixed some issues on the GUI launcher for 1.9.0. You can download the GUI updater from here.
This update is recommended for all FlightGear Mac OS X users. This update will fix the following issues:
- Fixed: archived aircraft couldn't be installed properly
- Fixed: some other minor bugs in installer (e.g. archived GUI-Updater was not installed properly)
- Fixed: FlightGear couldn't start if the path for FlightGear.app contains one or more white space. [...]