WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: View Of Transmission Case Vent
Courtesy of GENERAL MOTORS CORP.
Gently pry the vent (2) from the transmission case.
If the vent cap (1) comes off, gently pry the stem from the transmission case.