To awake Regigigas in Pokémon Black 2 and White 2 you need to have all the Regi's in your party when you interact with it. If you interact with it and you don't the Regi's in your party, it will say it's a statue and it will not wake up.
Regirock, Registeel and Regice:
If the player has all three legendary golems in their party, Regigigas can be awakened in Twist Mountain. It is not, however, necessary to use a trio caught in the Underground Ruins.
Regirock is available in Pokémon Black 2 and White 2, Registeel on the other hand is version exclusive in Black 2, while Regice is version exclusive in White 2. Despite Registeel and Regice being version exclusive you can actually get all the Regi's in one game, without having to trade or transfer the other Regi over from another game.
Regirock, Registeel and Regice can be found in Clay Tunnel, in the Underground Ruins, though only one Legendary Golem will appear at a time, depending on which key you have. Regirock is the default Pokémon in Pokémon Black 2 and White 2, it can be found in the Rock Peak Chamber in the Underground Ruins, which is opened when the player presses the A-button after walking 6 steps down and then 9 steps right, from the center of the panel just before the great door.
Upon catching Regirock, and not just defeating it or obtaining it from another game, Black 2 players are rewarded with the Iron Key and White 2 players with the Iceberg Key; these allow access to Registeel and Regice, respectively.
While Registeel and Regice are exclusive to Black 2 and White 2 respectively, players of the opposite version games can obtain their final legendary golem by transferring the appropriate key through the Unova Link.
Registeel requires the Iron Key to unlock the Iron Chamber, while Regice requires the Iceberg Key to unlock the Iceberg Chamber. The Iron Key can only be found in Black 2 and the Iceberg Key can only be found in White 2; the key that cannot be found in one version can be sent through Unova Link from another game.