ignore the instructions book and defrost it for at least 48 hours, behind the plate in the freezer is heat exchange, The more likey cause is the drain hole is frozen and that extra water has frozen over a period of time. The defrost heater only comes on for 20 minutes at a time.
Try this first b4 you look at the thermisters which moniter the temperture in freezer.
Hope this helps
foot
July 2009