Activating the Regi's

1. Fly to Pacidflog town. Make sure you possess the Pokémon Wailord and Relicanth before doing so; you can't open the sealed cavern that houses Regirock without them.
2. Surf west toward the fast currents. Stay near the bottom of the screen and go into the current; it will take you to a small island. Go into the current again, and it will take you to a pool of water that you can surf. Surf again, staying near the bottom of the screen, and it will carry you to yet another body of water that is surfable.
3. Go forward two more times, and you will be taken to a spot where you can dive. Use Dive to enter the cave. Go to the braille wall and read it. Use Dive again and you will be in another cave with braille writing on the wall.
4. Use Dig on the center of the wall and a door will appear. Go through that door, and read the braille on the wall there. It will tell you to move Relicanth to the first spot on your team, and Wailord to the last. Do this, and it will cause an earthquake. The game will tell you that it sounds like something has been opened somewhere. This means that you have opened the tombs of the Legendary Pokémon and can now capture Regirock.

Regirock in the desert

5. 
   
   Go to the sandstorm area next to Lavaridge. In the southeast corner of the desert, there will be a large rock with a door. Go through that door.
6. 
   
   Read the braille on the wall. Walk two steps to the right and two steps down. Use Strength and the wall will open. Walk into the room, and talk to Regirock to initiate the battle.
7. 
   
   Get Regirock's health as low as possible before attempting to capture him. Use status effects like Paralysis and Sleep to prevent Regirock from using Curse.
8.