did you pull up on the little knobs on the top of the left and right sides of the seat? if you broke those then im not sure what to tell you
Did you lock the seatback locks?
If you did, that would explain why the seatback would not release when you pulled on the knobs.
The cover for the seatback is held in place by a single long zipper. The zipper is removed at the end of the assembly process. You can pull the zipper apart, to gain access to the seatback release mechanism (make sure you unlock the seatback locks this time), but I don't know how your going to zipper the seatback cover back on when you're done.