Instructions
Finish the main storyline by defeating the Elite Four. This will unlock the Rainbow Wing.
Go to Pewter City and find the old man on top of a hill next to the Pokemon Center. Walk up to the old man and talk to him. He will give you the Rainbow Wing.
Go to Ecruteak City and find the Dance Hall. Enter the building north of the Dance Hall, and you will encounter the man blocking the stairs. If you have the Rainbow Wing, he will let you pass to enter the Bell Tower.