OK....here is your fixed CAPX
...and where do I start?!
First, if you give out a CAPX, pleas make sure, that if anyone is running it, it shows something. The first thing I had to do is to ahve your layout scroll to the place where the action is. Annoying <img src="smileys/smiley5.gif" border="0" align="middle">
Second, if you work with physics, you have to make sure that ALL objects that shall work together inherit the behaviour and have the right settings.
For example, and that was your problem, it is no use to have wheels set to "movable" and the car itself set to "immovable" - then it will just not move...
Depending on how realistic you want to have your movement, you can apply force to the car body or the wheels ....or you can apply torque to the wheels, which would be the most realistc way....
Ah, and one more - I deleted the 8-Dir behaviour from your catapult because it caused the flipping...