Light non-story spoilers
In most titles, booting up a base game with DLC installed would instantly queue up a sort of “radio signal,” or otherwise easily identifiable marker that says “this way to the DLC.”
That’s not the case for Horizon Zero Dawn: The Frozen Wilds, so here’s a quick guide to get you started.
Look at the top right of the map: that’s “The Cut”
Oh hey, it’s the expansion.
You can sort of wander in, but having the quest makes it so much easier. Just head to the fire near the Grave Hoard, then climb the mountain using the scripted bars. Run a little further north and you’ll find Yariki, which will lead you into the DLC.
Journey north to a new NPC and grab the quest
In case you get confused, here’s Yariki.
Or, get two other quests, which will also do the job
Ohtur in the Carja border fort (above), or Rhavid just beyond Meridian will also grant you access to the main DLC quest, if you aren’t able to approach the northeast portion of the map for any reason.
All three NPCs will lead you into the core new group that starts the Frozen Wilds questline.
That’s it! The questline starts at level 30, which is roughly where you needed to be for the end of the campaign. As always if you keep a number of different saves, you can select a previous version of Aloy that’s right around the “correct” level if that’s how you wish to enter the expansion.