When you are in China, go to the room where you find the elemental doors. As soon as you arrive, look up and look to the right a bit. There is a ledge that you have to get to. Go into the water element room. Once you get the element and destroy the enemies, you realize that the door that was locked when you went up the staircase is unlocked! Go through the door and go to the very end of the ledge. Scan the blue object and finally you will get the map to go to Serenity (the china online multiplayer place).