Instructions
Teach the "TM Dig" to one of your Pokemon. If you do not have it yet, go to Fallarbor Town and speak to the boy next to the tunnel entrance. He gives you the TM.
Place a Wailord at the front of your party and a Relicanth in the back. Relicanth are caught along routes 124 or 126, while Wailord evolves from a Wailmer, which is a common Pokemon.
Return to the sealed chamber and stand in front of the far back wall with the Braille on it.
Press "Start," then highlight "Pokemon" and press "A." Select the Pokemon you taught "Dig" to and then press "A" again. Highlight "Dig" and press "A" to use it. This allows you access to the final section of the sealed chamber.
Walk to the back of this chamber and then press "A" in front of the wall with the Braille writing. The screen will shake, and you receive a message that a door has opened far away. This means you are done with the sealed chamber and can now go hunt down the legendary Pokemon.