When Apple created iCloud to take over from MobileMe they initially used the @me.com names (again)
Later they issued @icloud.com IDs.
In iChat 6 and Messages the app(s) log in to Apple as well as AIM.
This adds a level of Security for the process that allows AIM to "read" Apple passwords.
The upshot is that iCloud issued @me.com and @icloud.com email IDs cannot be used in earlier versions of ichat from Version 5 down.
You also cannot access some of the AIM settings which does have some privacy issues.
At this point you would have to create an AIM name.
Use the link above and then use the "Get a Username" item top right.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
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