As I mentioned in the other thread about this, you should check out trueachievements.com. If you look up bioshock 2 and check out the boosting sessions you will probably find ones for the two metro dlc achievements. If none of the times or dates work for
you, you can always make your own session and probably won't have any trouble finding enough people to fill a room. Without making sure you're in a room with people that have the map pack, the chances of actually getting the maps are pretty slim unfortunately.