The Mystery Gift feature is used to receive special gifts online or using a promotional code in Pokemon Brilliant Diamond and Pokemon Shining Pearl. To unlock it, you’ll need to defeat the first gym leader, Roark, in Oreburgh City. Once that’s done head to Jubilife City and make your way to the Jubilife TV station. You’ll then want to head up to the third floor and speak with the Studio Director near the sitting area and agree to give your opinion. Choose the “EVERYONE HAPPY WI-FI CONNECTION” to become the proud new owner of Mystery Gift.