You need to get regigigas north of snowpoint city, you can get in there if you have been to the regigigas event, what I did however was migrated regirock, registeel and regice from my pokemon ruby GBA game (you can also do this for emerald and sappire), you can't do this on the new DSi. If you have all 3 regis in your party or have been to the regigigas event, go to the snowpoint temple and you will be aloud in. On the very bottom level there is a regigigas statue, push "A" and you will battle regigigas- be careful, regigigas is only level 1 so you might want a pokemon that knows false swipe. Once you catch regigigas, put him in you party. The 3 regi caves will open!!!