After getting Zekrom/ Reshiram, N will be at the Ferris Wheel in Nimbasa on a Friday. You need to visit him before you can go back to his castle at Victory Road (you'll also ride the Ferris Wheel with him), whereupon he will be there to battle you.
His team is dependent on the season your game is in, and you can battle him once per season.
N - subsequent battles
