User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-178.151.105.200-20130801105643/@comment-25496444-20130801111048

Supposedly it was to be released already, but it doesn't appear for me either. Maybe Steam had a crisis on the fact that it's the 3rd DLC and has troubles releasing it.