How I configured iChat to use the web proxy on Lancaster University’s Resnet. Quite easy, really. You’re welcome to refer to this article.
First of all, I opened iChat and chose Preferences from the iChat menu and clicked on the Accounts icon. Yours should look a bit like this.
If the Server Options is greyed out, go offline and open the preferences again. Check the box to enable Rendezvous messaging, it lets you talk directly with people who have iChat and are on resnet. Click on the button. Change the values in the dialog so it looks like this:
In particular, make sure the AIM Service port is set to 443 and the protocol is set to HTTPS. Click OK, close the window and connect!