Tamron lenses for dslr.is it auto focus?

I have here a tamron AF55-200mm f/4-5.6 ld macro lens, i'm aware that the AF stands for autofocus. But where i use it on my nikon dslr (d3100) it doesn't auto focus at all in any light or any setting, looking at pictures and reviews online i can see that the lens is auto focus and has a buton on it to switch between AF and M focus. But mine doesn't have this button. Have i been given the wrong lens? Or am i missing something?

Added (1). 10 points for anyone who can tell me if this lens would auto focus on my d3100:
http://www.jessops.com/online.store/products/73537/show.html

The Tamron does not have an autofocus motor - it will not focus with the D3100.
The lens is AF capable, but only with cameras that have a body AF motor, like the D80 - take it back and swap it for a Sigma.

What it means is that it will auto focus on camera bodies that have a focus motor in the BODY of the camera. The Nikon D3000, D3100 and D5000 do not have focus motors, so it will not autofocus on those.

You will have to focus it manually.

If you want autofocus lenses on your camera body, you need to buy lenses that have a motor in the LENS. These are marked AF-S (Nikon) or HSM (Sigma) - sorry not sure what Tamron's designation is.

That lens will autofocus only on bodies with an autofocus motor. The D3100 doesn't have one. Be very careful in buying lenses for your camera. You can only AF-S or AF-I Nikon lenses. Check this site for further details.

http://www.kenrockwell.com/...y-lens.htm