The first thing you should do is order the assembly manual for the 17 from Hobie Cat (you can do it from the Support section of this site). It will cost you about nine bucks.
Mast rotator arm / control:
OK, here are a couple of poor photos to go from:
Arrow number 3 is pointing to the mast rotator arm. It goes above the boom. There's a line tied to a cleat on top of the boom (you can see the outline of the cleat just ahead of the green part of the duffel bag in the lower left corner of the picture).
The line goes from the cleat, through the hole in the end of the rotator arm and back through the business part of the cleat. You control the amount of mast rotation relative to the boom by varying the length of the line.
Here's a marginally better photo of the set-up on a Hobie 14:
In this instance, the rotator is under
the boom, but everything else works the same. The rotator in this picture is pointed directly at the camera, so you can't really tell it sticks out about 10".