There's a tiny v-shaped metal tang or fork right at the top of the mast at the front, which catches onto a small bead that is crimped around the wire that's attached to the top of the main: this tang is the only thing that holds the main up. Loosen the rope that it attached to the end of this wire, so the rope is hanging free. Stand about 1 meter in front of the boat and pull the rope down (you'll feel the sail move up the mast), then move the rope forwards whilst still pulling down on the rope, so to get the bead out of the tang. Then let the rope out so the sail goes down: You may need to pull the sail down by hand if it is stuck.
Oh, and don't forget to loosen the downhaul / Cunningham first
in fact, it is probably best to lift the boom completely out of the mast track.When you put the main back up, do the reverse and make sure the bead goes back into the tang. Don't use the rope to hold the main up as you will stress your mast and the cleat.
There's a pic here http://www.hobiecat.com/forums/viewtopic.php?f=14&t=31980 of the set-up but yours probably wont have the white braid around the wire