Head to the temple at the northmost portion of Snowpoint City after getting the National Pokedex. Candice will then permit you to enter the temple. Just navigate your way to the different floors, picking up a CALCIUM and NEVERMELTICE along the way. Once you get to the room with a Pokemon statue in it, navigate the icy floor to the center of the room. Examine the Pokemon "statue" to battle Regigigas.
Note that in order to fight Regigigas, you need to have Regirock, Regice, and Registeel in your party. To get the 3 Pokemons, you can either transfer them from your Pokemon Sapphire, Emerald or Ruby GBA games, to Diamond or Pearl through Pal Park and capture them there.
Info about Regigigas:
Regigigas is a normal-type Pokemon, so take advantage of its weakness against fighting-type moves. Most of its attacks are of normal-type as well, and are pretty strong too. Superpower is a fighting-type move though. This Pokemon's attacks are powerful, but fortunately it also has the Slow Start ability which tips the scales to your favor for the first 5 turns of the battle. Make use of the 5 turns to lower your opponent's attack and speed stats. Moves like Charm and Scary Face work well for such purpose. Once the 5-turn duration ends Regigigas will get a great boost in speed and power, so hopefully you've managed to lower its stats enough beforehand.