To sign up for National Treasure, approach the Sign-Up Staff and register for the event. Once you have done this, go through the entrance to begin.
The event is set up into six different phases. During each phase, you must search through the room's treasure chests for a key to advance to the next room. Some chests will contain additional prizes, while other chests hide monsters. If you can get to the 5th phase, you will have a chance to challenge the 6th phase boss. After slaying the boss, you will obtain a special chest that you can open to get the event rewards.