Can someone tell me how to access the dlc after I've finished the game,

Do I need to access it before accepting one of the end-game quests?

Yes, you must do them before Hoover Dam.

You should have a quest to begin each of them in your quest log.

I was afraid of that.

Thanks anyway.

The game makes an autosave right before you start the very last quest. You should still be able to load it and do DLC.