He's dealing with one of the iMacs that the front comes off, not the back.
OP, what I've had great success doing to get that off is stick the credit card in at an angle, pointing basically towards the front corner of the machine, then once it's fully inserted, slide it towards the side of the case while straightening it out so it's now facing the front of the machine. The latch should pop open.
If you're still having trouble, you can lift the case from the bottom out until you can slide a small flathead screwdriver or a plastic spudger up from the front and hit the latch from that side. It's trickier, but if you're not having any luck from the back, it might help.
jW