After defeating the fifth gym leader,go to the Pokemon Center in Petalburg city. Then go inside the Pokemon Center. Talk to the man standing beside the PC. He will ask you for your new profile. Type in MYSTERY EVENT IS EXITING. The man will say,"You know the secret saying! So that means you are a member of the MYSTERY EVENTS CLUB. You will then be able to access the MYSTERY EVENTS icon in your main menu.