WARNING: This page is about a different car, the 2008 Pontiac G5 and 2008 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
Open the rear compartment.
Fig 1: Locating Outer Bumper On Compartment Lid
Courtesy of GENERAL MOTORS CORP.
Remove the outer bumper from the compartment lid by twisting the bumper counterclockwise.