Auto focus on nikon d3100?

I just recently (this weekend) got a new camera (nikon d80) and a lens AF nikkor 70-300 mm with a 62 mm focus I originally got the lens for my Nikon d3100 and it didn't auto focus but when i went to go put it on my Nikon d80 it auto focuses. My auto focus isn't broken because i used it all weekend with a different lens

Added (1). Is there a button on the d3100? There isn't one on the lens

Sure

The D3100 needs (motorised) AF-S lenses because it has no body motor (a cost-cutting measure).AF lenses don't work with it.

The D80 has a body motor and can use AF or AF-S lenses.

The selector should be on the left of the lens mount - somewhere below the flash button.

You have purchased an entry level dSLR and they do NOT have an auto-focus motor in the camera (just like Canon EOS cameras).In order to use auto-focus, you must use Nikkor AF-S lenses.

For now, just use the electronic range finder to manually focus your lens on the D3100

I'm surprised that you went from a D80 backward to an entry level camera. The replacement camera that followed the D80 was the D90 which the D7000 replaced.

My guess it that you did not do all the necessary research before you bought the D3100

The D3100 lacks a focusing motor that the D80 has.

It seems you didn't get the memo. The D3100 can only autofocus with AF-S and AF-I lenses. There's nothing wrong with your camera or lens.

http://www.kenrockwell.com/...s.htm#dslr

There's no such thing as a lens with a 62mm focus. Some lenses can focus to 62mm but they're called macro lenses. I'm sure you weren't referring to that.