you cant. use emerald or sapphire ,ruby for all the regis and there after go to snowpoint city to get regigas.
you can only get it through a link trade
Registeel is unobtainable from Pokemon Diamond and Pearl. You have to migrate it from Pokemon Ruby, Sapphire, or Emerald.
...You CAN'T!
you cant get registeele in Pokemon diamond
In order to catch Registeel in Pokemon diamond, you need to have gotten a level 100 Regigias from a mystery event. This mystery event has passed, so you can no longer get Registeel in Pokemon diamond, unless a friend trades you the mystery event Registeel. The only other way to get Registeel is to trade him from Pokemon emerald, ruby, or sapphire.
you can but i don't know how yet
you cant. you can only get registeel from Pokemon sapphire, emerald, and ruby.
You can find Regigigas in Snowpoint Temple in Pokemon Diamond, you will need to bring Regice, Registeel and Regirock for Regigigas to waken.
you don't need regigigas for registeel. you need reegisteel, regirock and regice for regigigas on Pokemon pearl or diamond
Registeel,Regice,and Regirock are all Pokemon your required to migrate from their respective game(s).
You can get Dialga Mesprit Uxie Azelf Giratina and if you somehow get regirock, regiice, and registeel on to diamond you can also get regigigas
you need regice, regirock, and registeel in your team when you fight him
You can only get a regice, regirock, and a registeel by migrating them from ruby or Sapphire. If you migrtate these Pokemon you can also catch regigigas.