This sounds like a stupid question, but I don't see any way to start playing the DLC content.
I bought the game a long time ago, and I think I finished the main scenario but I'm not quite sure now. For sure I have never played with the DLC, despite having bought it a long time ago as well.
Do I have to make sure that I did indeed finish the game and beat the last boss?
In the game menu, where you have play-parameters-quit, I see a DLC box below, and the mouse cursor does change when hovering over, but it doesn't do anything if I click.
If I run the game from the DLC store page, it starts the vanilla game as well.