WARNING: This page is about a different car, the 2010 Porsche Cayenne, 2009 Porsche Cayenne, and 2008 Porsche Cayenne. However, it is still accessible from the selected car via links, so may be relevant.
Centre the vent with the ring markings -3-
pointing upwards and press it in -Arrow-
until the locking tabs can be felt to engage.