How to get my sigma 18-200 DC to autofocus on my Nikon d5100?

I recently purchased the sigma 18-200 DC. How do I get it to auto focus? On the manual there's a photo of the lens with a part at the bottom that has the MF/AF switch, is that an adapter I need to purchase?
I use the Nikon d5100.

You should've bought the HSM version - it has the built-in motor necessary for autofocus on Nikon's cheaper bodies.

I assume your lens does not have an AF motor. Your camera for sure doesn't have one too. There's no way your lens will autofocus with your camera. Go back to the store you bought the lens from and ask if you can have it replaced with one that can autofocus with your camera. You will surely have to add more cash to do so.

You don't, that's what happens when you buy a manual focus lens and use it on a camera without a built in focus motor.

All the three answers are right.d5100 does not have a af motor to rotate the lense rings to achieve focus. Still you can manually focus. Buy nikon lenses with signatures like af s sw which will work with any non motorised camera