Here's what I had to do to get this to work and this will work for Stone Prisoner, Wardens Keep, and Return to Ostagar
First go to http://dragonage.wikia.com/wiki/Downloadable_content_(Origins)
once there at the top look for Dragon age series tab which will open a new tab and look for DLC click on that and chose Dragon Age Origins
At the very bottom of the page that comes up is all the DLC
Here's the important part there is the DLC and then there's the offer which is where the quest begins so in regards to Wardens Keep it puts Levy in your camp to start it.
Look for this at the bottom of the page, or if you can get these links to work here
DAO_PRC_OCP_1 - Offer file for Return To Ostagar
There is typically one way to install these and that's C: Program Files (x86)>Origin Games>Dragon Age>Bin Ship>daupdater and dragging the datzip you downloaded into it and clicking install once you highlight it.
I think with Ostagar I had to do it this way I can't remember but if the DLC is installed right you should see a file in your offering of each DLC
After you have downloaded the offering of the DLC, go to Start if you have Windows 7 open documents>Bioware>Dragon Age>Offerings
Hope this helps
