adding auto-installer to older wordpress installations?

I’d like to add the auto-installer feature to my wordpress installation. I added it using Fantastico more than a year ago. I’ve noticed that later installations allow you to upgrade with one click in Fantastico, but with older installations, all you see is a “only manual upgrade is possible.” How do I upgrade my old installations so that they have the auto-upgrade feature with fantastico? By auto upgrade I mean all you do is click a little link that says “upgrade to 2.1.”