Treyarch has announced that they have removed the Nuk3town only playlist and have replaced it with a Chaos Moshpit playlist. Nuk3town playlist is for special weekends only.
FYI: Nuketown-only playlist weekend has ended. Nuketown now available in Bonus > Chaos Moshpit for those who have downloaded the map.
— Treyarch Studios (@Treyarch) November 10, 2015
Chaos Moshpit is available under the Bonus category. This playlist features a collection of small multiplayer maps, including Nuk3town. In order to view and play this playlist, Nuk3town map must be downloaded.