How to enable 3rd mouse button under wheel?

Hi

I have been searching the forum for specific info about this but have not come to an answer.

I am working on a mac Mini from 2007 here that has a standard Dell PC mouse and keyboard attached to it (the ones that follow when you buy Dell PCs). The keyboard works fine and the mouse too, the problem that I am having is that the 3rd mouse button located under the scroll wheel is not active on OSX. The scroll wheel is working fine. When I go to mouse preferences I don't see anything there to enable this mouse button.

The os is OSX Snow Leopard.

Any ideas? I would like to assign functions to this 3rd button under the wheel like i.e. closing tabs in Chrome, like it works in Windows.

Thanks.