I've released the GUI launcher updater for 1.9.1, which is available from here.
The updater fixes the following issues:
- FlightGear might not be invoked due to misconfiguration of FG_ROOT (data directory)
- Some options like aircraft and airport are not properly parsed, and c172p and KSFO are used even though you specified other aircraft or airport.
Please install it from Advanced Features >> Others >> Install Add-ons from the GUI Launcher. And then restart FlightGear.app.[...]