The mast receiver tube threads into the deck area and rests over a "nub" down inside the hull (Yes, opposite that screw on the underside).
Fit a long blade (large flat blade) down inside the mast receiver. Simply unscrew from the hull. Check the length (they vary and the part is one length for all, custom cut) of the damaged part and cut the new one to the correct length. Screw back into the hull seeing that the base of the part fits over the "nub" in the keel.
Part is # 84550001 $3.50 List
Director of Parts and Accessory Sales
Hobie Cat USA