in pokemon platinum, how do i open my mystery gift?

No you got it wrong. It’s true that you can get a gift during an event but it has to be a Wi-Fi Event to get it over Wi-Fi. The event you’re talking about is probably an event where you have to go somewhere to get it (like Toys R Us or Gamestop), to get that kind of gift you need to go to that place where the event is being held and get the gift via Wireless Connection, not Wi-Fi.
You need to check and see what kind of events are there, don’t confuse Wi-Fi Events with topic events.

Oh and one more thing, the Member’s Card was given as a Wi-Fi Event a LOOOONG time ago, if anyone told you that it’s being held as an event he was lying, the event is over now.
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Yes, today there is a Pokemon event, but it is not over Wi-Fi. The Jirachi giveaway is being held at participating GameStop stores, and the Member’s Card event has been done with since September. If you want the Jirachi, go to any participating GameStop Store and follow these instructions:

Bring your DS system and a copy of Diamond, Pearl, or Platinum to a participating GameStop store. Once you’re in the building, turn on your DS, go to the menu screen, and make sure you have two or less wonder card in your Mystery Gift, because receiving a gift will add a wonder card, and the maximum number of wonder card you can have is three. Once that’s checked, select "MYSTERY GIFT", "RECEIVE GIFT", then "GET VIA WIRELESS". After downloading the Pokemon, go to any PokeMart and talk to the delivery man in green to receive your gift. Make sure you have 5 Pokemon or less in your party when picking up your Pokemon.

Check out these website periodically for news on upcoming Pokemon events:
http://pokemon.com
http://pokebeach.com
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You must have that Wi-Fi to open the Mystery Gift or it says get a gift from friends you will need your Friends Code.And then it says,get gift via wireless you will need a Wi-Fi to all the mystery gift.
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