i downloaded all three of the current dlc's but decided to complete the main story line before getting on to one of them (dead money), does this mean i have to restart the game or load an old same to play it, on fallout 3 i got the dlc's after completing the game and it just continued so im abit confused, if anyone could help me id appreciate it alot, thanx

There will be no DLCs set after the game has ended, nor will there be any DLCs like Broken Steel, extending the end of the game.

You can't play after you did the last mission...that's why everyone has a save before you play the last mission.