Change the spawn co-ordinates of the core you added to the game (Atlantis doesn't exist in any Standard, or released client, so you must have added that yourself?) to match the co-ordinates at which you placed your Atlantis (beta Ricarten) map.
Seems pretty obvious to me, but I never added a new field core to a client, and I can think of at least two ways I may go about doing that. Of course, the way to change the co-ordinates would be completely different depending on which method I chose. (as would securing the client against player abuse after adding a new spawn)