Never leave a kayak on a cart longer than it takes to move to and from the water. This will distort the hull bottom.
Sorry that the instructions for the clamps were not passed along to you with the cart. Dealers should be able to properly instruct you on their use. The hull should rest on the pad. The clamps should take any further load to the cart. Tip the hull upside down. Slide the cart into the scuppers until the pad rests on the keel. Adjust the clamps to rest on the scuppers. 3 points of contact are required.
Cart and Clamp instructions:
http://static.hobiecat.com/web_uploads/2010/12/15/80044201.pdfScupper Cart Use Warning:
http://www.hobiecat.com/forums/viewtopic.php?f=21&t=33397