you cant you can only migrate it from other games, sorry!
Sorry dude, you can't get registeel in platinum without a cheat. But you can catch Regigigas in Platinum, but you'll need all three of the Regis to get him.
If You Have The 12th Movie Regigigas In Your Party,Registeel Can Be Found At Iron Island.
that doesnt make sense
you need REGIROCK, REGICE, AND REGISTEEL.
beat elite 4
Regice: mt coronet exit regirock:sand storm place registeel: iron island order: Regirock regice registeel
regeggias in snowpoint temple with regirock regiice and registeel on platinum only
You can only catch Registeel in Platinum if you have the Japanese movie or Toys 'R' Us event Regigigas. Registeel is hidden inside a secret cave within Iron Island if you have the proper Regigigas to unlock it. The cave is near the exit. If you don't have the specific Regigigas then you have to migrate Registeel from Ruby/Sapphire/Emerald.
To get Registeel you would have gotten the level 100 Regegigas from a wirerless event or migrate it from saphire, ruby, or emerald.
you have to have them traded from ruby Sapphire or emerald
you must have regirock, registeel, regiice
the end of iron cave. see platinum.com for more details