Thank you for replying. I started a new game and activated broken steel before the ending. It worked.
No it does not require you to restart your game!
Update Fallout 3 to 1.5
When you start a convo with that brotherhood of steel girl (I don't remember names) on the ending of the last Fallout 3 quest (Project Impurity), you will get a popup confirming the DLC has been activated.
I decided you sacrifice myself in the game, so instead of the game getting finished, you wake up at the Brotherhood of Steel HQ.