Well, you see, once you finish the game, you'll be back at your house. When you come out, your rival/friend will come with his sister. The little girl will tell you she had visions of Zoroark calling for you at Victory Road. You fly there. It'll be smart to fly to the top, then make your way to Zoroark, and you follow it.
You enter N's Castle, but you wont be able to capture that Zoroark, unfortunately. You find N as you go in. He will follow you, and at the end, you'll find a place where you battled N (if you have B/W ) the first time. He will have a Reshiram/Zekrom LV. 70.
Once you defeat either one, N gives you the Reshiram/Zekrom in a stone. Then you go to Dragonspiral Tower, go to the very top and Reshiram/Zekrom will form and you can capture it.