Firstly, Welcome
If you are using large 3ds or x files as a level, to actually detect collision with every part of it would involve using "set object collision to polygons". This is SLOW and currently crashes many PCs

You could convert to BSP but many people are also having trouble getting this to work.
The best way to detct for collisions with specific parts would be to build primitives and shape them where you want the collisions t5o take place, then make them invisible, but checl for collision with them rather than the actual world

Its a long way round I'm afraid, otherwise try BSP, or see if the polygon collision works on your PC. Just remember it might not work on some1 elses just yet