There are two menus on this page. One at the top and the slightly lower one headed "iChat 4 How-Tos"
This menu contains links to Setting up a Screen Name*, the Start Up Screens and so on up to Video Chats
These have changed some what since I wrote that page.
Apple still issue IDs via iCloud now rather than MobileMe
These are still Valid AIM Screen Names and will work in iChat 6 (Lion OS X 10.6.x) and Mountain Lion (OS X 10.8.x) with Messages.
However these two version have a second Log in to a server called Me.com which in turn then allows the AIM servers to "See" the password held at Apple.
Effectively it means iCloud issued @me.com and @icloud.com Names will not work in iChat 5, iChat 4 or earlier.
Also the "Name" part is essentially know to AIM so you cannot use the email itself to "create" an AIM Name
AIM will accept you registering and email as the AIM name as the @ is a valid character or a plain Username (No @AIM or AOL suffix needed)
You can also use Jabber IDs
GoogleMail accounts can have "Talk" enabled in the account settings and the ID can then be used as a Googletalk ID.
Facebook also run a Jabber server and you have to enable "Chat" on your settings there.
There are also many other Public Jabber servers you could join (mostly registration is though a Jabber specific app than the created ID is used in iChat)
I have linked you to my website about iChat at the iChat 4 How-to point.
The broader strokes of Getting a Screen Name are that it is an AIM or AIM Valid Name or Jabber ID that is needed (many people use both)
However since I wrote that page Apple's MobileMe has come to and end and names are now issued through iCloud.
Initial iCloud IDs ended @me.com (like MobileMe did) and now tend to be @icloud.com name
These ARE valid AIM names but will only work in iChat 6 and Messages (Lion and Mountain Lion) due to the way these password checking process works between AIM and Apple.
This in turn means any iCloud Issued name will not work in iChat 5 or earlier versions.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously