How do I create a game that is controlled by a plug-in USB game controller? I already have the scripting done for the game itself. I just want to know how I would script it to be controlled by a USB game controller.

I haven't tried it but check these:

1: http://blog.touchmypixel.com/2008/04/joystick-joypad-in-flash/

the above recommends -> http://www.autohotkey.com/

2: http://www.bytearray.org/?p=64

3:http://www.as3gamegears.com/input/gamepad/

recommends -> https://github.com/iainlobb/Gamepad