Hi LordChrinox,
What exactly do you mean with that Steam does not detect the DLC? When you launch the game, a launcher should show up that enables you to check the dlc's that you've bought. Further, I'd like to see a bit more information on the system you are trying to access the game (MAC, Win, Linux + what version) and what version of the game you bought.
If you are running on Windows and are not seeing the dlc in the launcher, please try the following:
First, download and run
Visual C++ 2010.
Second, download and run
Visual C++ 2010 SP1.
When that is done, go to
Windows Update and make sure you have downloaded and installed
all .NET updates Windows Update gives you.