How can I download the DLC i was promised?

Looks like it may have been pulled off the site for some reason. I am sure it will be back before too long. You are not missing out on anything much yet unless you really want mutton chop sideburns. :)

I would ask support if it is unavailable for you.

Maybe the DLC is there, I just simply can't find it :/

There have been some problems today with the DLC links and they vanished for a time. Seems they're available again now. You can find the links to the installers on the right side in the goodies section of your W3 page.

Lol I'm sorry I have no idea where the "goodies" section is

right side, opposite game d/l files.

If you are using Galaxy, it's under the game, then the second button next to play and extras.
They don't show up there though; you have to go find them in the folder where extras are stored.

Go to your GOG-Library and click on Witcher 3. The game page that opens has 2 rows. The left row is titled "Game Downloads" where you should see the main installer and patches. The right side is titled "Game Goodies". That's where the links for the DLC installers currently reside, at least for me. I'm describing the regular site btw, since I don't use GOG Galaxy.

As Heratim said,it disappeared for a while.The changes at Gog need time to be fixed.

So... I "bought" the free dlc through the GOG installer and did not ever see an option to download it. Is this step broken? It says I own the dlc but I don't have any files for it, and I don't know how to get them.