After defeating Clair and finishing her task to go to dragon's den, go to professor elm's house. He will say that a few kimono girls are waiting for you at the ecruteak city theatre. After reaching there, you'd be forced to fight the five of them, each holding different eevee evolutions. After winning, they would proceed to the tin tower. Climb the tin tower to find them waiting at the top and you can talk to them to summon ho-oh!
