I've got something strange, when I tried to use a model as a levelpart.

This is a model I did in Cinema4D and I wanted to use it as a cave. I exported it to 3ds, imported it in MED and saved it as MDL. When I add it to my game, my player always falls through it.
Then I added a block as bottom, but now he can only walk to the half of the model and can't get any further. Does anybody know what I did wrong?