How do you disable AP Isolation

I know that this may be a simple question for many of you, but I am having some issues with my ATV, and I think it may be related to my AirPort Extreme. I would like to disable AP Isolation on the Airport Extreme, but I don't know how. Can anyone give me the steps (for a newb) on how to do this. Or point me to an article/post that does?

Thank you!