Point your browser to
http://extremeui.equinoxgaming.com/ and see if you connect/get the log/update note page. This is the page that the AUPC displays when you are on the Extreme tab.
It's almost sounding like something is blocking you from this site. Make sure if any firewalls are activated that the AUCP has access to access the web. All it should need is http port 80.
Also
Is this new computer on the same hub & router the other computers are on? Does the updater work on the other computers?