He is located at N's Castle.
First, you need to see him at the Ferris Wheel in Nimbasa City on Friday. After riding it with him, you can meet him at his castle, which is connected to Victory Road. Remember where the Zoroark was, blocking your path? It will now lead you to his castle.
N will be inside. Go all the way up to battle his Reshiram.
N then reappear after you catch Reshiram on Dragonspiral Tower. After that, he will be at his Castle once a season to battle you with weather teams.
Skip to 3:30.