I not sure what you mean by "enable" it. If you just plug a monitor into it the screen should come up, plug in two monitors and each will be a clone. In window you go to the screen properties and you can extend your desktop onto whichever is considered the second monitor, that gives you one big desktop instead of two windows showing the same thing.
The DVI out is always on as long as there is something plugged into it. Are you using just the DVI or both DVI and VGA? Does the VGA work? What monitor(s) are you using?
-spike