black 2
you've got to beat the Pokemon league and then your rival will tell you about a zoarark on victory road, so follow it and it will lead you to N's castle. You will then have to battle N (level 70 zekrom/reshiram) and if you beat him Zekrom is all yours
To get Zekrom in Black 2,you have to beat N first.Then he will release his Zekrom.Go to Dragonspiral tower,battle it and Catch it if you want.
Source : I have White 2 and the same thing happened with Reshiram