Hi there: I'm afraid this is going to be a bunch of words.
The expansion reads a laundry list of conditionals from the main game and so the safest way to play it, reactivity wise, is to start a new game.
That said, the expansion should appear if you've completed the main game.
It specifically unlocks if you have had the final Fran conversation where she tells you to "Go to Hell". The travel pamphlet for Hell should then appear in the bottom righthand corner of the main menu. This should be true even if you experienced this conversation in your current game before updating to get expansion content. (Note: you need to select Start Game one last time to see this conversation with Fran).
If you're not seeing the pamphlet, try going back into Start Game. If the pamphlet still isn't showing up, please let me know.
Sorry for the wall of text. Hopefully this helps. <3
Ben