(NOTE: Zekrom is only available in Pokemon Black 2. If you have White 2 you'll have to trade for it.)
The legendary pokemon of Pokemon White, Zekrom is back for another round in Pokemon Black and White 2. In fact, it's somewhat integral to the plot, though the part it plays is admittedly small and comes right at the end. Nevertheless, you can catch this pokemon as before - though it's a bit trickier to do this time around.
You can't catch Zekrom for your team until after you've defeated Ghetsis and made your way to Victory Road. On your way up Victory Road, towards the Pokemon League, you'll have to climb a mountain face. If you watch the left side of the mountain you'll see a Zoroark standing in front of a cave. Chase the Zoroark into the cave and follow it down into the ruins of Team Plasma's old castle. Inside, in his throne room, you'll find N waiting. He'll present Zekrom. Fight and beat it. Don't bother trying to capture it, that won't work.
Once you've trounced Zekrom, N will reduce it to a stone that will look familiar to players of the old games. Take this stone to Dragonspiral Tower, north of Icirrus City, and climb to the town of the tower. Once there the stone will come to life and return Zekrom to its original form. Now you can battle and capture the pokemon.