Where to find Zekrom; Reshiram
If you are playing Pokemon White, you will capture Zekrom, the Black Yang Pokemon. And vise versa; in Pokemon Black, you will capture Reshiram, the White Ying Pokemon. Either one, depending on the version you play, can be found at N's Castle when confronting him.
"Make me your ally and we will fight together!"
In this case, you will have a easy chance to capture it. All you need to do is use Hypnosis or Sleep Powder and throw a Dusk Ball at it. Congratulations. You caught it in only two turns. Keep in mind that making Pokemon sleep (asides Rest) makes it very effective to capture.