I have a big problem. The whole scene of my bowling game is inside a cave, which is a .mdl file. But I can't apply physics within the cave. Pins are scattering here and there, ball is moving here and there. If anyone know any solution, please help me the earliest possible time.

If your cave is a MDL file, then, no matter what the shape of the model, it will have a Box collision detection scheme; a box bounding box. Hence, when you are "inside" the cave, you are "inside" the objects collision hull and the Physics Engine doesnt' know what to do and simply goes crazy.

That's my guess. You should strive to make the cave out of a hollow cube or some other method.