First you have to find all the other regi's and he will be in a cave i dont remember what but it has ice in it and u have to slide to him and press 'A' on him and he will start moving.
go to Snowpoint city with the regis (regirock, registeel + regice. (obtainable in previous gameboy games, then upload them.)) in your party to unlock the secret cave where regigigas is.
tips
lvl 70! tricky one. train regis to about lvl 70-80 as they have to be in your party. not sure what type but save before attempting- then if it goes wrong just switch off and try again!!!
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you need event regigigas
Trade with Pokemon Platinum, Pearl, or Diamond.
you gotta get all regis for regigigas and the event for arceus is over.
You need relicanth and wailord to catch the regis in sapphire/ruby/emerald. You cannot catch the three regis in platinum unless you have the event Regigigas (this event has now ended). If you have the special regigigas in your party, three new caves will open up in Sinnoh and you will be able to catch the three regis.
You cannot catch any of the Regis (Regirock, Regice, Registeel and Regigigas) in Pokémon HeartGold or SoulSilver.