That's not the correct place to create a new iChat AIM account -- that place is for creating new system accounts so that other users can log on and use the computer. The "Accounts" pane of the System Preferences has nothing to do with iChat.
Open iChat, then select "Preferences..." from the "iChat" menu in the menubar. Under the "Accounts" pane, you can add an AIM account there by clicking the plus sign ("+") under the list of accounts (you'll probably see the "Bonjour" account already listed there).
When adding an AIM account there, you will need to know your AIM screen name and the password associated with that screen name. If you do not have an AIM account yet, then you can sign up for one from
http://www.aim.com (click "Get a Screen Name" in the upper right-hand corner of the page).