How to get 400 Days for the retail PC-DVD version?

I've purchased and played through the retail version, a DVD containing all five episodes. It is not a Steam version thankfully, but I can't find a way to buy and install the 400 Days DLC. In the game's Download Content, it only shows the five episodes, 2-5 marked as installed.

So how / where can I purchase the DLC for this version of the game?


This discussion has been closed.