Remove the following 3 screws:

Two 7.5 mm Phillips from either end of the left I/O frame.

One 9 mm Phillips from the middle of the left I/O frame.

In some models, all three of these screws may be 7.5 mm.

Remove the small black plastic spacer at the bottom of the left I/O frame.

Some models such as the Early 2008 may not have a removable spacer at this location.