Zoroark blocks you because you have to beat the Pokemon League. After you beat the Pokemon League your Rival / Friend's sister will mention a Zoroark at Victory Road. Go back to the Zoroark where finally you will encounter N. If you beat N then he will give you the Dark Stone. Take the Dark stone to the top of Dragon Spiral tower where you can Catch Zekrom / Reshiram.
Hope this helps ^.^