OK, I'll give you the steps:
1. Go on Mystery Gift. (In some games, you need a password to get it)
2. Choose the option that says (or is close to saying) "Receive gift through Wi-Fi."
3. Wait for it to connect.
4. Your event gift should br sent into your game.
Note: You must have a wireless router in order for it to occur. It also must be compatible with the DS.