At the bottom of the carto is the connection (with the slots on the sides and hole in it) use a small paper clip (or the like) and GENTLY pull up on on this middle piece through the hole, it will lift the connection. Only slight adjustments are needed, but be careful! There are fragile things within the hole. And like all holes you don't want to just cram stuff in there- easy... Easy! Yeah- Like that...Oh,Oh, God! (Sorry! I got carried away.)
Seriously this really works. Juiceblaster sells Carto Keys for this very purpose. Hope this helps!