First thing I would check is that you have 'linked' the car to collide with your rigid body by checking the little boxes next to it.

So select the car - then check the box next to the rigid body you want it to collide with (like you do for the ground/gravity/etc)

If this isn't working it's likely to be the way ou have made your model. If the faces are the wrong way, you will have issues.

Try posting your model (sketchup file zipped up) and .3dr file here if you get stuck so we can help you further.