You have to remove the air filters and black plastic cover that runs across in front of the windshield. I haven't taken my x3 apart to be able to give you specific directions but I have taken my 530i (e60) apart quite a bit. There should be a couple screws near the front on either corners and then there will be a bunch of plastic lock nuts that you have to turn to unlock.

__________________
2004 BMW 530i
2008 BMW X3 3.0i