Hi,
The Original Post says nothing about trying this in Mountain Lion and Messages.
I have an external to Apple email that I made an Apple ID some years ago.
When iCloud was introduced I linked to iCloud and was given the @me.com Address I had.
Months later I had an email about being given an @iCloud.com ID as well.
In Mail If I set up the @me.com as an account and then send to the @iCloud ID I get the email.
If I use any of the IDs (and the password that works with all of them) then I can access my iCloud web page, my Apple ID (although this does not list all the options of emails).
In Messages as the iMessages Account each name/ID seems to be treated as a separate ID.
As an AIM Login in iChat 6 or Messages the IDs (well the @me.com and @icloud.com) work as separate IDs again.
You are right in that iChat 5 has the Add account drop down that lists @mac.com @me.com (As in MobileMe) and AIM.
Essentially iCloud comes along for Mountain Lion and Messages rather than iChat 6 which probably means that it has an Add Screen similar to the iChat 5 one.
This does mean adding just the Username and relying on the Software to add the @me.com part.
Alternatively you can enter the whole Apple ID as an AIM name.
Check the ID in Keychain Access (Applications/Utilities) and double click the entry then use the Access tab to check iChat and IMAgent are allowed to access the info. (it may be sert to allow Any app but if restricted make sure iChat and the IMAgent are allowed.)
Visit the Apple ID site and check the emails linked to the Apple ID are showing as Verified.
The password being longer than the AIM allowed 16 characters is the most common cause.
I am not sure if AIM allow the same characters as Apple (all my AIM and Apple ID passwords are upper and lower case or numerical characters).
The next most common cause is that the Keychain item has become restricted but does not list the right apps.
After that we are in the realms of there being something up with the account at Apple itself.
10:21 PM Sunday; July 7, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
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
Message was edited by: Ralph Johns (UK)