Here is the GPSP keys.txt details:

GAMEPAD
-------
Up		XAsix -
Down		XAsix +
Left		YAsix -
Right		YAsix +
A		Button 1
B		Button 2
Start		Button 3
Select		Button 4
L		Button 5
R		Button 6

I believe that select and start would then be left trigger and left button, but it might be different depending on xboxdrv. Try some combinations and see what you can get, alternatively you can use the keyboard and press F10 to enter the menu, then adjust settings from there.