Instructions
"Pokemon Diamond, Pearl &Platinum"
Travel to Jubilife City and visit the television station. Talk to the character on the third floor that does the interviews.
Input the words "Everyone" and "Happy" after he asks the first question. Input the words "Wi-Fi" and "Connection" in response to his second question. The interviewer will inform you that the mystery gift can be found on the main menu. The phrase is different for each version of the game in a different language.
Exit the game and return to the title screen. Scroll down the menu until the "Mystery Gift" option is highlighted. Use the stylus to tap the "Get mystery gift" option, then "receive via wireless."
Check the "news and events" section of the Nintendo website to see if there are any upcoming nearby events where you can make use of the mystery gift function. If there are no events, the process can be done with a friend by selecting the "Get from friend" option under the "Get mystery gift" menu.
"Pokemon Gold, Silver &Crystal"
Travel to the city of Goldenrod and visit the department store there.
Go up to the fifth floor of the department store and speak to the girl wearing green. The "Mystery Gift" option will now be available from the startup menu.
Select the "Mystery Gift" option from the main menu while in infrared range of a friend doing the same. Both players will receive a mystery gift.