The DLC adds a new chapter onto the end of the game. This is story related. If you have beaten the game already you can load and beat the last chapter to get into the new chapter.
After you have entered the door you will unlock the achievement 'Beyond the Gate'. Follow the path to the very end and you will unlock 'I Am Become Death' and then let the credits finish to get 'The Wheel Turns'.
This will take around 5 minutes.