Correct, pulling the knob should open it.
Are you sure you're pulling on the correct knob? At least on my model, it is the small knob on the upper left. Unlike many TLRs, the back is hinged at the bottom, not the top, so the back opens the opposite direction of most. When you do open the back, be careful about the positioning of the focusing lever. The back can hit it when it swings down.
I did get metal Minolta lens cap with my Autocord but I'm not a big fan of the design. One end locks in but the other is on a hinge and can come undone. If the other side comes undone, the whole cap can fall off. Mine came off this weekend while in a bag. I'm seriously lucky the metal cap didn't scratch the lens. I'll probably pick up another cheap plastic one on ebay soon.