Like most things there is an alternative way of doing this.
Basically it is using the method to set up a New Account (Screen Name) when you have not actually set one up in the first place as it is possible to Click through the Start Up Screens.
Go to the iChat Menu and select Preferences.
The next window will show a range of icons across the Toolbar and you want the Accounts one to be Highlighted/selected. This is a smaller pane than in iChat 1.
Where this picture shows my Screen Name is where you need to concentrate on. If you have not got a Screen Name entered you should be able to type in this Box and the Server Settings button should also be dark/active text. The Password box should be empty if you have not set anything up yet.
You can set up more than one Screen Name and use this same spot to change between them. The arrow at the end of the Screen Name field will drop down into a List if you have more names, as the Picture above shows. If you plan to swap between two or more names, do not use the Clear option that appears in here.
So if you have got here with no Screen Name entered in the Start Up Screens enter the Information now. Leaving the Password blank will get iChat to prompt you each time you try to Log in to AIM. Otherwise it is stored in the Keychain like other passwords.
The Server Settings Button
This is available to use when you are Logged out of AIM. The picture is an iChat 1 one and in iChat 2 the Proxy info is hidden at first with a drop down option.
When you click on the Server Settings Button it will show you the Server name and the TCP port that iChat is using to Log in to that server with. The default server is login.oscar.aol.com and the port is 5190. (There are no longer any other servers available)
The port can be changed. The reason for this is that iChat uses Port 5190 on the TCP protocol for Logging in and sending basic Text Chats and also on the UDP protocol for File Sending and Direct chats and some modems and Routers that have to use Port Forwarding do not like this dual use - so changing the port here is useful. (Port Forwarding only allows a Port to be forwarded once to one IP and some devices insist and list Protocols as well which makes it seem the port is to be forwarded twice)
From testing it seems that the AIM servers will actually accept a login on any port (there are, in fact, 65535 of them). The suggestion is to use one below 1024 as these ports are always open in a routing modem or router. On the whole we recommend using port 443 as this is used by some mail applications and secure Websites and will also be open in the Mac Firewall in versions up to Tiger (10.4.x).
If you use a Proxy server to get on to the Internet this is also where it is set, although generally iChat seems to do better with use of the System wide Proxies set in the System Preferences > Network pane.
Next - How to Add Buddies