You have to get the event Regigigas. The event was about 2 years ago.
You must get it from a trade, or migrate it from a gba game.
Unfortunately, the only way to acquire it is through either Pal Park, or trading it from Platinum. You can't catch it in the wild.
If you have the event Regigigas, then you go to Iron Island and you will find the Iron Ruins inside. Then, catch Registeel. He should be lvl. 30.
you have to migrate it from sapphire/ruby/emerald
apparently u need a toys r us regigigas but i got regiigas by having 3 regis in snow point temple but the 3 regis ain't mine but they didnt show up even though i visited there caves my moshi monster is popitss
in Pokemon ruby there are these legendaries: Groudon, rayquasa, regice, regirock, regesteel, latios. (these are without cheats or events)
You can either use action replay or it might not work but I'm not sure about the other ways
first have 5 ultra balls then go to MT ember on one island and search on green in front of MT ember P.S i have 20 magmar
to catch regesteel without a master ball or ultra ball you need a timer ball, it is a ball that after 25 moves or so catches the Pokemon automatically. i suggest using the move growl or splash for 20 turns that's when the ball will be most effective. Yeah and to warn the users i just want to say that it might take a few tries even after 20 turns. and you can also try to put the Pokemon asleep.
In order to get Registeel you will need to have either Black 2 and the iron key unlocked or have a friend who has it. Once you have the key, send it over Unova Link to White 2 and activate to unlock the Iron Chamber.
first of all, Pokemon is gay, and second of all, who the hell was dumn enough to invent pkoemon anyway? Pokemon is for gay guys, nerds, geeks, whatever name you wana call it. =)
Platinum