WARNING: This page is about a different car, the 2002 BMW Z3 and 2001 BMW Z3. However, it is still accessible from the selected car via links, so may be relevant.
Unfasten vent screw on expansion tank.
Fig 1: Removing Vent Screw On Expansion Tank
Courtesy of BMW OF NORTH AMERICA, INC.
All bleeding occurs through vent plug on expansion tank.
Cooling system is vented after the filling procedure.