How do I install a blade on my circular saw?

I was recently given an 18 volt Black and Decker circular saw. It came with a blade but not installed on it. I am having trouble getting the bolt out so that I can install the blade. Is there something that I am missing? I read the manual and it said to just give a quick "jerk" in the counter clockwise direction which I have done numerous times to no avail. Please help!