How do I find the regi trio in black 2?

Question

I managed to get through Clay tunnel, I found the room where Regigigas is and where one of the Regi's is but I have no idea what to do

Answer 1

You'll notice in the main menu where you change the difficulty in Unova Link, there's an option with different keys. By activating the respective keys you can make the Regis appear in the Underground Ruins, which is within Clay Tunnel.

Registeel requires the Iron Key, which is exclusive to Black 2.
Regice requires the Iceberg Key, which is exclusive to White 2.
I believe Regirock is in both versions.

However, like you're able to exchange difficulties via Unova Link, the keys can be exchanged between versions.

Answer 2

After you get Regirock, you will get the Iron Key. Reeneter the tunnel after activating the Iron Key on the Unova Link menu. After you do that, use the Unova Link's Key Link to transfer over the Iceberg Key, and catch Regice by activating the Iceberg Key and entering the Tunnel again.