Forward stick is gas, backstick is brakes.
See this for checking your joystick setup, or
this if you are trying to use mouse as a controler.
Also note:
In controller mapper if you have "auto select mode at start of flight" checked, but do not have anything mapped for mode 2 (vehicles), you'll have no vehicle controls. Copy mode 1 (there's a button for it) over to mode 2 and problem should be fixed.
