You need to do this:
After completion of the game, you will encounter N within the ruins of his castle. While there, he will challenge you to a battle, and when he loses, his Pokémon will convert back into a stone. Take the stone to Dragonspiral Tower and you'll be able to capture it. Zekrom is exclusive to Pokémon Black 2 while Reshiram is exclusive to Pokémon White 2
Having not been able to capture it when first encountered, after you have captured the Reshiram or Zekrom owned by N, you can return to the Giant Chasm to find Kyurem. Kyurem will be in its native form when captured, but will leave behind the DNA Link item which will allow for the changing of form
To fuse them just "Use" the DNA link.