You have to have a certain amount of badges..I dont think you have to have all of them, im
not sure.

get every pokemon

ihave pokemon pearl but u have to beat leage

beating the elite four. then fly to snowpoint city go to the lady outside the temple and
talk to her. She won't let you go in but candice, the gym leader, will.

Hope this helped

you have to get the national pokedex first

After you get the national poke dex you go to snow point temple

I bet you need to get Regice,Regrock,and Regsteel.And I have a Question how do you write
questions?

There is a pokemon called Regigigous in it so you cant get evry pokemon ok.