WARNING: This page is about a different car, the 2010 BMW 535xi, 2009 BMW 535xi, and 2008 BMW 535xi. However, it is still accessible from the selected car via links, so may be relevant.
Necessary preliminary work:
Open the floating headlining.
Raise slide/tilt sunroof.
Press water channel (1) upward at the marked points by pressing firmly with your thumb.