Claudiub, I PM'ed you the details, but I'll post it here anyway. You need to add retrofit code +AHMU to the vehicle order, so the car knows that it has the trailermodule (assuming that you have installed the trailer module). Adding this retrofit code will code all the individual modules for you including adding the zoom mode to the camera.
If the dealer is doing it, see page 12 here
http://www.kneb.net/bmw/E70/Trailer%...-%20towbar.pdf