Pull up on the red strap. You have to lock the sides in place before pushing the bottom down. There is a hinge on each side of the button. Just pull up on each side, making sure the hinge on each side of the button engages (both have to engage or the side won't stay rigid so you can't really mess it up). Once all 4 sides are straight, push down on the floor part.
ETA: You don't need to press the button until you are breaking it down.