First, you have to be at least level 18. Next, use a saved game that is post Magran's Teeth or Mapping the Archepelago (you must have completed He Waits in Fire). Lastly, and very importantly, you have to return to your ship and sail the seas. That's where the DLC starts.

I hope you're not playing this on PoTD w/ permadeath options. B/c there are things that will kill you or your companions d.e.a.d. I've taken to saving a lot before fights! Loving it, though.

Don't bother accessing it. Tedious not-fun dungeon crawling with broken enemies. Boring and a sad finale to Deadfire.

If you like reloading combats over and over again because enemies do broken things, then go right ahead...