Welcome to PF regan1,
mg cannot be equal to rcos(theta) simply on dimensional grounds. The former is a force, and the latter is a length. Could you post the equations that you are having trouble with more carefully and in greater detail?
For a banked curve, if you look at the road in crosssection, it is essentially like an inclined plane, with the "downhill" direction being the direction towards the centre of the circle. In the absence of friction, the only two forces that act on the car are gravity, and the normal force from contact with the road, which can contribute to the centripetal force. This should be helpful to you as a starting point for working out the force balance equations.
