It should have a pin in place to hold down your latch on the ball. They make a diferent type lock for this type of latch. It is a long nail looking device and a little bolt lock. Just remove the pin and thread the nail looking peice all the way through then slide the little lock on the other side of it. I had to ask the dealer on mine cause I couldnt figure it out either. Here is a link to BPS of the lock type I am talkin about hope this helps. you can pick them up at any marine store also.
http://www.basspro.com/webapp/wcs/st...=SearchResults