Once you get the egg that the Sazuka Leader tells you about, go to Divermon's Lake. Go down the ladder at the entrance of the lake. You will see Divermon in the water. Talk to him and he will give you a Divermon card. Go to the square platform and press Action. When you are in the water, keep going to the right. You will see light. Go to it and press X. You will go up and a message reading "Welcome To Dueling Island" will appear.