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.
Press in lock (1) before installing fast-release coupling.
Fig 1: Pressing In Lock
Courtesy of BMW OF NORTH AMERICA, INC.
Push on hose.
Fast-release coupling must audibly snap into place.