The base game for this DLC is free and you will need to add it before you can add the DLC I believe.
IMPORTANT:
Click the following link and click install in the upper right corner: https://steamdb.info/app/1576340
Alternatively put steam://install/1545820 in your browser.
Do NOT go to normal Steam page as it is not free there.
I put the normal page into main link but for info ONLY and because I am not allowed to put in the direct link.
Credit and thanks to Kaizuken.
Enjoy!
"Limited Steam accounts can not sign in to SteamDB.
Your Steam profile must be set up, and have spent 5$ on Steam."