First of all, I want to clear up that cinema4d objects are very large in size.
and the object you imported to the game is not transparent.
The reason why it looks transparent is because of the size.. its so big, that the camera goes inside the object, therefor it looks transparent.

Try to scale the object in C4D before exporting.

If this doesn't work, then the problem is not the scale, but the cull mode.
Here are the different cull modes:
-none
-cw
-ccw

The none means there are no cull mode (trans.).
CW means the side that you see is trans., that way you will see inside the object instead of outside.
CCW means the side you don't see is trans.

If the first solution didn't work then you'd need to change the cull mode. from CW to NONE.