Here's how you can get it in those games:
1. Find the three Regi statues: In the "Ancient Tomb" located in the southern part of the map, you'll find three statues that need to be activated.
2. Activate the statues: To activate each statue, you need to use the "Regi" to activate it. There are three "Regi" you'll need to find:
* Regirock: Located in the "Cave of Origin" in the northern part of the map.
* Regice: Located on the "Seafloor Cavern" to the west of the "Ancient Tomb".
* Registeel: Located in the "Iron Island" to the west of the "Ancient Tomb".
3. Activate the Registeel statue: After activating the first two statues, you can activate the Registeel statue. This will cause Registeel to appear.
4. Catch Registeel: Once it appears, you can use a Pokéball to try to catch it. Be prepared for a difficult battle.
Important Note: In order to access the "Ancient Tomb" and the other locations, you need to have at least the "National Pokédex" and have defeated the Elite Four.