Brennie,
If you're talking about the black slider lever, my memory is that it just pops in or out; however, to put it in, you have to find the orientation of the sleeve it goes into (I usually just poke around a bit till I feel it go into the sleeve).
A related question for the group: if it were necessary to remove the chrome vent, how is that done with minimal breakage to the metal tabs (?) that hold it in place?
-David