I'm not sure how far into the story the room actually unlocks, but it's pretty easy to find.
When standing at the the Level Select Globe in the control room on the Watchtower, run directly to the right of the area to find a giant door that should open as you approach it.
Once you go inside, you'll be back in the room with the giant statues of Superman, Batman and Wonder Woman from earlier in the game. Green Loontern, Kevin Smith, Adam West and Conan O'Brien will all be standing in the center of the room "partying".
The achievement pops as soon as you enter the room.
Incidentally, when you reach 100% Completion, this room will also have the 100% LEGO Stud Fountain for the "Bats all folks" achievement.