AF Nikkor 80-200mm f/2.8 - Nikon ED?

I just purchased this lens for my Nikon D3100, and for some reason, it won't auto-focus. Can someone please help me out. I know the auto-focus on the lens isn't broken, because when I tried it on my friend's camera (Nikon D2X) - it worked just fine. I also tried it on a friend's D300s, and that worked as fine as well.

Am I don't something wrong? Is there a certain setting that I need to put my camera on? Or, does this lens just NOT work with my camera?

This lens will not autofocus with the D3100.It relies on the camera's autofocus motor, which the D3100 lacks. The lens would autofocus with a D90 or D7000.To autofocus the D3100 would need a lens that has its own motor like the Nikon AF-S lenses (which this one is not).