They are not in this version of Indigo yet. You could get it in by hacking, but you may get banned if you do that.
So far registeel is not in Pokemon indigo unless you use firebug.
You get Regigigas in Team Shadows Room along with Regice and Regirock, Registeel is not in it unless you use a cheat code :)
well first you need to release all of your Pokemon and then get a registeel.
You can't find registeel in Pokemon HeartGold.
You can find regigigas in the snowpoint temple, but you'll need regirock,regice,registeel to battle it, it will be at lvl70
You can't find Registeel in Pokemon Platnium. You just migrate it in Pokemon Ruby, Pokemon Sapphire, and Pokemon Emerald.
...You CAN'T!
by the mountain
Entei can be found in Nimbasa City in Pokemon Indigo.
cheek seribrii.net
Yes, there is every Pokemon on Indigo. The easiest way to find Pokemon is to go to the ROOMS.
I doubt you will find Giratina on Pokemon indigo as it is not a generation 1 Pokemon (it isn't one of the first 151 Pokemon species)