by fortree city go down then to the right then up
You can't find registeel in Pokemon HeartGold.
You can obtain a Registeel by trading it from Ruby, Sapphire, and Emerald.
You dont find registeel on D or P. U have to transfer him from Ruby, Sappire, Emerald.
...You CAN'T!
by the mountain
You find registeel in the forest area near fortree city. Note: You need to complete the puzzle of the sealed chamber before you can find him, just like the others.)
route 228
cheek seribrii.net
You can't find Registeel in Pokemon Platnium. You just migrate it in Pokemon Ruby, Pokemon Sapphire, and Pokemon Emerald.
To get Registeel you would have gotten the level 100 Regegigas from a wirerless event or migrate it from saphire, ruby, or emerald.
Registeel is located at Chroma Ruins north from the room where you capture Spiritomb and you find a steel statue which requires cut (x5), capture Gallade and go back to Chroma Ruins where that steel statue is and use Cut on the steel statue and you get a Registeel
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.