Hi Lewis, What do you mean you don't see the model; do you mean when you run FlightGear you aren't seeing it in the aircraft list?
To install an aircraft on Mac OSX you simply go into your applications folder in Finder, right click on FlightGear.app, select *show package contents* and go to Contents>Resources>fgdata>Aircraft .. and place your aircraft folder you downloaded into it. .zip or .rar files need to be unzipped first and then placed in there. There is no installation required on Mac, just drag and drop and your flightgear will automatically read it on relaunch.
If it is a different issue please provide more information and terminal errors.