How can you open the mystery gift in platinum?

Answer 1

Once the Mystery Gift is activated, walk/run/bike to the nearest Poke-Mart. Once you walk inside, you will see a guy in green standing next to the counter. Walk up and talk to him. Once you're finished talking, he will hand you a package. If it's a Pokemon, the Pokemon will join your party. If it is a mystery gift where you're handed an item, like the Secret Key or Oak's Letter, your character will store it in the Key Items pouch of your bag. That's how you open the Mystery Gift.