You can only become a tower if you use the fsx multiplayer option. If you join on line flight similator groups such as Virtual Skies, Ivao and Vatsim you will need to download a standalone application that works with the Fllight Sim package
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There are various ways in which you can play FSX. You can play just with a keyboard and mouse. However I don't recommend using the arrows keys to control your airplane. If you don't have a joystick, getting one will be a great idea. In the meantime you can use the mouse. You can also play in FSX multiplayer and fly with others that live around the world in real time, as well as talk to them via a microphone.
You cannot
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No, but you can download planes with 'airstairs' (Stairs)