1.) edit ~/.supertux

2.) insert

;; joystick number (-1 means no joystick):
(joystick 0)
(joystick-x 0)
(joystick-y 1)
(joystick-a 3)
(joystick-b 2)
(joystick-start 0)
(joystick-deadzone 4096)

3.) done