WARNING: This page is about a different car, the 2011 BMW 750xi, 2011 BMW 750Lxi, 2010 BMW 750xi, and 2010 BMW 750Lxi. However, it is still accessible from the selected car via links, so may be relevant.
Stand vehicle on a level surface and secure against rolling off.
Undo oil filler plug (1).
Add automatic transmission fluid according to instructions in the BMW diagnosis system.