@GoCanes: I didn't purchase it earlier on. So that probably isn't it.
However, as my fiancee was too impatient to wait for this to be solved she bought me the normal game and the DLC as well.
All is good now. Although I think there is a bug with creating buildings. For some reason sometimes buildings are not accepted as such so it seems, even after building it properly.
I am now stuck in Prison story 3 where I have built a solitary confiment cell block but it's not recognised by the game as such. Where do I report this bug?