Find the the Outcast Outpost. If you put *Aiding the Outcasts" as your active quest it should give you a quest marker.
I have a feeling you may have not received the quest at all. If your Fallout 3 folder is on a separate drive from where your GFWL downloads are, your DLCs will not load. The solution is to either move the DLC esm/bsa files into the Data directory in the Fallout 3 install folder (which from personal experience has led to many issues with the game randomly freezing and hanging) or reinstall Fallout 3 into the same drive as GFWL downloads (C: if your Windows directory is on C:).