You can't, unless you have the event Regigas distributed during the game's launch at Toys R Us. If you don't, then you will need to import them from Gen III games (Sapphire, Emerald, Ruby)
Yes.
you cant get all the regis only regigigas.
You have to migrate from EMRALD SAPHIRE OR RUBY on to Platinum :)
you get the regis in emerald ruby or sappaire those are the only way
get movie regigigas.
migrate a s s
No.
to find regigigas in Pokemon platinum you need to trade over the regis from another game and go to the bottom of snowpoint temple with the regis in your team. you will see a regigigas statue and it will come to life if you have the regis in your team.
just get the three regis, then go to the temple in snowpoint city.
national pokedex
you migrate them from ruby Sapphire or emerald
You have to have a special Regigigas from an event.