slimm50, to control vehicles, you have to have Mode2 mapped in OPTIONS > CONTROLS > Map Controller. You can usuallly just COPY MODE 1 (except a few functions like gear raise/lower instead of flaps and damage display, which need to be remapped). Also, make sure you have the box "autoselect mode at start of flight" checked - otherwise you need to assign keys to the select mode1 (planes) and select mode2 (GV) functions.
To acellerate, move stick forward, left and right to steer.
Note that in PT boats, the accelleration is controlled by the throttle axis, not X axis.
If you have any more trouble, feel free to email me or contact me in game.