Instructions
Travel to Route 129 and catch a Wailord. This Pokemon is rare to catch, so this may require time and patience.
Go to the underwater cave near Soothopolis City to catch a Relicanth. This is also a rare Pokemon to catch and may require time and patience.
Select Relicanth as the first Pokemon in your party and select Wailord as the last.
Travel to Pacifidlog City by using the "Fly" ability.
Travel over the water to the south west of Pacifidlog City by using the "Surf" ability.
Find the small island in the water near Route 134.
Stand in the center of the island, then surf left a short distance. Stop surfing before you reach the water currents, or you will be carried away with the currents and will have to start over. If you have done this correctly, you should be above the Sealed Chamber.
Enter the Sealed Chamber. In the first room you should see Braille slates on the floor.
Stand on the slate in the middle and use the "Dig" ability. This will open a secret room.
Enter the secret room and follow the path until you see another Braille slate on the floor.
Stand on the slate and press the "A" button. This completes the puzzle that will open the door to the cave where you can catch Registeel.
Leave the Sealed Chamber and use the "Fly" ability to go to any city with a Pokecenter. You can change the Pokemon in your party since you no longer need Wailord, Relicanth, or the Pokemon with the "Surf" and "Dig" ability.
Travel to Lilycove City using the "Fly" ability.
Walk to the far left in the city until you find a set of stairs. Climbing the stairs and following the path leads to the Ancient Tomb.
Stand in the center of the tomb and use the "Fly" ability to open a secret door. Entering this door will take you to the room where you can catch Registeel.