To my knowledge there is no button. There is a small recess at the back top edge that you slide your finger in and pull to open. Gently! If you get a clunk when you pull and the filler door stops, your vacuum operated lid lock is in the locked position. Unlock the car and try again. If the filler lid is locked with the car unlocked try started the car to 'charge' the vacuum system. If this does not work you will have to open the trunk to access the lock, and manually push the locking pin back.