If you are looking for a free flight simulator for MAC then I suggest you to go for FlightGear. FlightGear is an open-source, cross-platform, flight simulator. The main aim of this FlightGear project is to create a sophisticated flight simulator framework for use in research or academic environments, for the development and pursuit of other interesting flight simulation ideas, and as an end-user application. You can select an Airport and then an Aircraft to fly it. The airport that you select here will be the start point of your flight.
The latest release gives more fun and flexible flight to every Mac user. It runs on both Mac OS 10.4 (Tiger) and 10.5 (Leopard). The latest version comes with a new fancy GUI launcher that eases your flight. You can easily change the aircraft, airport and other options.