Mass Effect 3's Extended Cut DLC will be available for all three platforms tomorrow (at varying times, of course), but if you've beaten Mass Effect 3 and left your saves untouched, you're ready to start the fight up all over again and see the new content.
Simply load the autosave that sets you down as you enter The Cerberus Base.
.... Jeez. That's like... a minimum of around three hours of gameplay. Do I even give a shit about this anymore?