You get Regirock by default. Just follow the video. After the battle, you get the Iron Key in B2 and the Iceberg Key in W2.
When you have the key, go to your key system and switch the key from Regirock to Registeel. Then do the same puzzle (stand in the middle of the circle, 6 steps down, 9 steps to the right). Go inside to face Registeel.
Then, find someone with W2, trade keys with them, and repeat the steps above to get Regice.