Good news everyone. The issue has been fixed:
Our apologies to anyone affected but the Dark Souls ll Old Iron King DLC is now free to download for season pass holders. — PlayStation UK (@PlayStationUK) August 28, 2014
That's a pain. According to a statement on the Facebook page they're working on a enabling a link so PlayStation season pass holders can get their stuff.
"Crown of the Old Iron King, the second chapter in its The Lost Crowns DLC trilogy for DARK SOULS II has not been linked to the PlayStation®Network season pass yet. This new DLC only appears has a DLC to purchase. For those who have purchased the season pass, please do not purchase the DLC: We are working on enabling the link so you can download the second DLC automatically on your console. We are sorry for the inconvenience and ask you for a little bit more patience to enjoy the obscurest and most wicked pits in the dark souls II world."