I finished the main Dawnguard story and am now following up with this quest. The problem is that I can't figure out where I'm supposed to go inside the ruins. I've followed all the paths available in the area, including meeting Katria, but it seems I'm supposed to go somewhere and I can't see what the path is.

Anyone done this that can get me on the right path?

Where exactly are you? Have you just met Katria and you can't figure out where to go after that? If you look towards the water you should be able to see a column forming a path over to a small piece of land that has a pipe leading into a tunnel. If you're past this, then you gotta give more details.

Edit: Here's a link to a walkthrough: http://www.youtube.com/watch?v=-376xkE75U4&feature=related

Thanks for the link. It looks like I missed Katria's comment about hitting the thing to open the next door since I don't have subtitles on.