in the ruin macanics cave i dont know where its is but you find it in the ruin macanic's cave
There is no 'platnium', but you can find it on wild pokemon.
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No, you'll have to trade it from Platnium. Which in Platnium you find it in the Rock Peak Ruins.
Honey Trees
in girantinas cave
in the trophey garden
you can find one with a traner on 216
You can't find Registeel in Pokemon Platnium. You just migrate it in Pokemon Ruby, Pokemon Sapphire, and Pokemon Emerald.
beat every trainer
you have to trade him from Pokemon platnium
use your map to find out
go to the suburban route on the pokewalker.