You can't. You can get it only through migrating, trading or cheating.
If You Have The 12th Movie Regigigas In Your Party,Registeel Can Be Found At Iron Island.
beat elite 4
If you have Regice, Regirock, and Registeel.
regeggias in snowpoint temple with regirock regiice and registeel on platinum only
you have to have them traded from ruby Sapphire or emerald
you must have regirock, registeel, regiice
You need to have Regice, Regirock and Registeel in your team.
You have to have regice, regirock and registeel. Then go to the snowpoint temple.
Regice: mt coronet exit regirock:sand storm place registeel: iron island order: Regirock regice registeel
you need to have regirock, regice, and registeel
To get Registeel you would have gotten the level 100 Regegigas from a wirerless event or migrate it from saphire, ruby, or emerald.
If you have the Regigigas from the Toys'R'Us, you can take it to Iron Island to have it unlock a secret room in where Registeel will be awakened. It is level 30 as well.