The lens mount on the camera has a little black tab sticking out forward from a moving spring-loaded ring running around the mount. The tab is at about 2:00 on the mount, as you face the camera. With the oldest lenses, the back of the lens fouls on this tab, preventing the lens from going on. If you try too hard, you're going to break off the tab! Next to the tab there should be a little silver button you push to permit you to flip the tab out of the way so that the lens will mount and not foul on the tab.
Ah, beat by a couple of minutes, while I was writing