When I tried to login with my jabber id in the current version of Adium "=" is not a valid character in input field of the "Edit Account" window (Error: The characters you're entering are not valid for an account name on this service.). According to http://trac.adiumx.com/wiki/CreatingJabberAccounts "=" is a valid character in JIDs and also in all other clients for linux/windows I used before my JID worked ;)
I checked out the latest version of the Adium source from SVN and saw that "=" is not white-listed as a valid character in ESJabberService.m
A simple patch would be to add "=" in line 100 and 116 to the list of allowed characters in ESJabberServices.m