Hi, I'm new to this. I have one of those heavy duty Swingline staplers and I'm having a big time problem trying to add more staples. Any tricks or suggestions? I'm assuming the silver square thing on the back needs to be unhooked somehow, but no go. Is there a release button somewhere I haven't found? HELP! I need to get more staples put in. This particular stapler can take 1/4", 3/8" and 1/2" staples.

Thanks.