I just bought the DLC, so I do what the sticky post on these forums tell me to do and load my game before the end game content, it crashes to desktop after a few mintues, so I figure maybe I need to load a later save, it works for about 5-10mintues I get no quest pop up as I read on the forums then crashes to desktop. I try this with many saves even changing gfx options in a hope to fix it but it seems to be the DLC itself that has corrupt the game as yesterday before I installed the DLC everything was running smooth (other than the usual random crashes due to game engine stability) but that's something I have come to expect from this game and deal with. But this is disgusting that I cannot use any of my save games with this DLC no matter what I do so I guess I have to start again from the beginning but I'm not going to do that, because of the principle of it. I know I won't get a refund I will most likely be ignored or told to start the game again. I just wanted to give my story in a hope that these issues will be fixed. Peace.

Try turning off Steam, then turning it back on and redownloading the DLC. I had the same problem, and this is what worked for me.