Return to the Pokemon League and then head south onto Victory Road. Enter the nearby cave and keep following the path until you find the two Veterans. South of them is a small staircase leading down. Take it until you see the Zoroark in the bottom-right corner of the screen, which is where you should head on over to.
Follow the path through the cave until you wind up on the other end, where you'll run into the Zoroark again. If you came this way before beating the game, that Zoroark stood in the way of the cave there, but now that you've beaten it, it'll lead you inside.
Inside, it will lead you over to a staircase which takes you down to N's Castle, in ruins and everything.