Instructions
Travel to any participating GameStop in your area and download the special Pokemon to your copy of "Pokemon HeartGold" or "Pokemon SoulSilver."
Use another DS to transfer the special Pokemon from your copy of "Pokemon HeartGold" or "Pokemon SoulSilver" to "Pokemon Black" or "Pokemon White." Use the transfer center in "Pokemon Black" or "Pokemon White" to initiate the procedure.
Travel to the trailer outside of the Lostlorn Forest with one of the three legendary dog Pokemon in your party. A person will appear outside of the trailer. The person is actually Zoroark in disguise.
Talk to the person to initiate the Zoroark battle. Use Fighting-type Pokemon to weaken Zoroark, and then use a Pokeball to capture it. You can access this event only once, so make sure to capture it. If you do not, you can reset your game and try again.