The hidden celebrity party is located in the Justice League Watchtower hub.
From the Control Room, stand at the galaxy map/level select. Then simply head to the right and through the giant door. You will enter a room with Kevin Smith, Adam West, Conan O'Brien, and the Green Loontern. The achievement will unlock as soon as you enter this room.
This is also where the stud fountain is located after hitting 100% completion.
Note: Shabobble, adds that you will be able to access this room upon completing the story mission "The Lantern Menace". (Thanks for the info)