When you get into Dragon's Den in Blackthorn City (you need to defeat the Gym Leader Claire first to get in) Go down (you might have to battle someone) and keep going down, then when you come to the other end of the bridge go to the far left/right of it and go through the Whirlpool by using Whirlpool (Duh.) Then go to the front of the shrine, jump onto the platform and go through the entrance.
