Instructions
Go into any Pokemon Mart. Pokemon Marts are typically found in cities and towns.
Walk up to the counter, facing the paper in the middle of it. Press the "A" button. A "Questionnaire" screen opens.
Highlight the first blank and then scroll through the list of words that appear. Choose the word "Link."
Highlight the second word, located to the right of the first word. Select "Together" for the second word.
Select "With" for the third word, directly under "Link," and choose "All" for the fourth word. Select "OK." The store manager notifies you that Mystery Gift is unlocked.
Save your game, quit and then restart the game. At the title screen, select "Mystery Gift" to access the feature.