Those white bits either side are the heavy duty anti slips, they screw sideways into the ali rafter and they should be hard against the bottom of the glass. Get your fingers under each side of the rafter cap, pull apart to release the teeth and pull up to unpeel the rafter cap from the ali, the glass won't fall out so don't worry, afterwards the caps just thump down to clamp the glass down onto the gasket. See its all simple when someone talks you through it