Those legendary Pokemon are exclusive to Pokemon Black and White, and their respective sequels, Black 2 and White 2.
However, there are a few ways you can get them in your X and Y game:
* Trading: You can trade with someone who has Zekrom or Reshiram from another game.
* Pokemon Bank: If you have a Pokemon Black, White, Black 2, or White 2 game, you can transfer them to Pokemon Bank and then to Pokemon X or Y via Poke Transporter.
* Mystery Gift: Occasionally, Nintendo will offer events where you can receive Zekrom or Reshiram via Mystery Gift. However, these events are rare and usually time-limited.
So, while you can't catch them directly in Pokemon X and Y, you can still add them to your collection through these methods.