Log in to the Donder Hiroba website. On the Top page, choose "大会挑戦状" (Tournament Challenge) in the light blue box.
Under "公式大会" (Official Tournament), the third purple heading from the top, choose "開催中一覧" (Current Event List).
Read and agree to the Tournament Rules by checking the "大会ルールに同意します" (I agree to the Tournament Rules) checkbox, then pressing the "参加する" (Sign Up) button.