How to play fallout 3 broken steel add on?

You continue to explore the area and you’ll run into new characters and missions.

And no, you don’t have to start a new game.

well my last save is at project purity. I start there and cant leave. Do I need to go further back?

Check your radio stations. There should be a new one. Or something in the mission screen.

You need to beat fallout 3 all the way up until the part you “die”.

“SPOILERS”
Your character does not actually die from the radiation, and that is when the expansion begins.

(And make sure to get you a set of enclave fire armor.)

ok I did beat it, I “died”, then there was a cutscene. And after that, when I try to continue it just takes me back to right before I die.

Did you enable the addon in the Launcher window?
I don’t have it in front of me, but there should be a settings thing and in there an option to put a checkmark on downloadable content.