Atari800macx+MacBook Pro

I installed this emulator together with USB Overdrive on this mac, and i plugged in the gravis gamepad pro.
I succeeded in mapping the start , select and reset keys to the controller. But i have no idea how to map the fire button and the direction keys for the gamepad. I'm a bit puzzled with all these options in the prefs panel of the emulator .
Can someone point me in the right direction and tell me which settings i need in the emulator and overdrive so i can finally blast these aliens out of the sky with my moving gun ??:msn_shy: