The change in acts is preceded by some narration from the Death Knight. When you exited the Citadel,you entered act 2. The Battlefield keys you have now are for act 2 battlefields. Act 1 battlefields are closed to you. Since the battlefields are just for trading and level boosting, use them if you feel you need to. My opinion is that they make the game just a little to easy if you exploit them too much.