first you should have beaten the elite four and have the national pokedex once u have that
done you go to snowpoint city and go to the temple once ur in go to the last floor
 once your there you well see regigigas but u have to go through the ice once ur past the
ice and in front of regigigas you cant battle him if you dont have regirock regice and
registeel once you have them with you you press a on regigigas then your able to catch im
... good luck 


p.s its best if you use a master ball