Selector Lever Spring Loaded Button
WARNING: This page is about a different car, the 2002 Mercury Cougar. However, it is still accessible from the selected car via links, so may be relevant.
Courtesy of FORD MOTOR CO.
To move the lever from park position and to engage "R", "2", or "P", the ignition lock must be set to "II", the foot brake must be depressed and the spring loaded side button pressed in.