Do you mean the rails themselves (front to back) or the crossbars?
If you don't have rails, you can purchase them and either install them yourself or have a shop do it. It's an ordeal. The headliner has to be dropped to get to the bolts.
How come you don't have rails? I thought the X5M was the only one where it was optional.
Here is the DIY:
http://www.xoutpost.com/bmw-sav-foru...stall-diy.html