HI,
So this particular @mac.com address is also linked to an @me.com iCloud Address ?
And you are using the @mac.com ID in Messages.
On the other front about which name the iMessages part uses in Messages go to the Preferences > Accounts
In the top item which should be the iMessage "Account" (even though it does not give you a Buddy list).
In the right hand pane it should list the accounts logged in then at the bottom give you a list of IDs that can be logged in (there is most likely only one there at present)
In Messages beta it seemed that you had to add a second before you could delete the first.
You will also probably have to log the Account out first (Untick the "Enable this Account" box)
NOTES.
You seem to be saying you have an @mac.com and @me.com name linked to iCloud
(You will find you can log in to your iCloud web page with either name and the same password - this is also true of several other Logins to Apple such as the App Store and iTunes as well as here at Support Communities)
However the two "Names" will be recognised as two separate AIM valid Screen Names that you can use with Messages.
The password for this Apple ID account should be kept to no more than 16 characters as this is the AIM Limit.
Also due to the way iChat 6 in Lion and Messages login to both AIM and to me.com the @me.com name will only work in ichat 6 and Messages (And not iChat 5 or earlier)
The Dual Login allows the AIM servers to "see" the passwords only when the me.com login is done by the app.
You can then add them separately in Messages > Preferences > Accounts > Add Button (New Pane/window) > AIM chosen in the top drop down.
9:42 AM Friday; July 27, 2012
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