Configuring @me.com account on Adium
Officially you cannot use third party clients for connecting to AIM with iCloud account. It seems that the protocol is the same, but authentication is different.


  AOL/Apple changed the authentication method for new iCloud accounts. This will have to be fixed in libpurple before it'll work again in Adium.


This quote is from this board.

Try using a nightly build of Adium, if it's fixed — it's there.