Well, first I do not see any built in cam for the ZZ stitch. I think it will need a ZZ cam on the cam drive.
Second you have to rotate one of the knobs off of the straight stitch position to engage the ZZ drive. And there is I believe a lock knob or lever to hold it where you want it.
Some control has to move the big silver lever to the rear so that it can pivot the black cam follower into the cam. That would be one of the knobs.
I don't see anything missing that's obvious.
The white tab is a stop for something, probably the big silver thingy.
Joe