I've never used a caulk gun, so maybe the answer is obvious, but how do you get the plunger pulled back? I can't get any of the parts to move.

There is a small rectangular tab where the plunger rod goes into the gun. That tab has to be pushed to release the plunger and allow it to be pulled back. Once pulled back the cartridge can be inserted. The small tab locks the rod in the pushing position so pressure is maintained on the caulk tube