There is 2 regigigas you can get in platinum. One from an event and then you have to get all three regis in caves. After all that is done hatch an egg like a shinx or something that is from an egg. Train it up about to lv3. Get the three regis in your party (you don't have to migrate) at Snowpoint city, then go to that temple, enter it and then go round untill you are right next to him.
Put your level 3 Pokemon at the front of your party and then talk to the Level 1 (seriously) Regigigas and make sure you have at least 10 dusk balls.
Added by: Theodagreat Mar 22nd 2009, ID#10569