WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Data Link Connector (DLC)
Courtesy of GENERAL MOTORS CORP.
Align the DLC to the I/P trim pad.
Slide the DLC upward in order to engage the tab (1).