you have to migrate it in from ruby sapphire or emerald
...You CAN'T!
Registeel is unobtainable from Pokemon Diamond and Pearl. You have to migrate it from Pokemon Ruby, Sapphire, or Emerald.
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 cant get registeele in Pokemon diamond
No.
you bring it to snowpoint temple
you can but i don't know how yet
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.
regice regirock and registeel can only be obtained in diamond/pearl using action replay or migrating from gba
Okay, unfortunately, you can't get Registeel on Pokémon Diamond, or Pearl. You will have to migrate Registeel from Pokémon Sapphire or Ruby (possibly Emerald). Hope I helped-PokéFanAs a side note, there may be an extremely rare chance that you find someone that has Pokemon diamond AND a Movie Regigigas. If they transfer the movie regigigas over to their platinum version, they can capture regirock, registeel and regice, and therefore allowing you to trade with this person.
yes
you need regice, regirock, and registeel in your team when you fight him