Wait for someone to put one up for trade and simply search for it. If you have the requirements of the trade offer, it will then allow you to trade.
Also note you WILL have to have seen a Zekrom to be able to search for it.
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Unless you cheat, you can only obtain Reshiram in Pokemon black. You will need to trade to Pokemon black if you have white for zekrom to be in Pokemon black.
There is no code you trade with someone who has Pokemon white
No, it is a version exclusive to Pokemon White. You'll have to trade it from someone who has that version.
You go to the Pokemon center. Right next to the globe, there's a girl who asks you to do wireless communications. then say global trade
Actually you will not be able to find a Groudon in pokemon black or white. It's because you will not be able to find a Groudon in the wild. The only way you can get Groudon in pokemon black and white you will have to trade with someone who has a Groudon or you can use Global trade.