Like Generation II, Mystery Gift offers a means for players to externally enhance their adventures in the local region. With FireRed and LeafGreen there is a degree of legwork involved to get the Mystery Gift running, but the rewards are far greater this time around. Similar to the Mystery Event from Ruby and Sapphire, you have to enter a secret phrase to unlock the feature.
To access Mystery Gift, go to any Poke Mart in Kanto or the Sevii Islands and find the clipboard on the main counter. From here, enter the phrase "LINK TOGETHER WITH ALL" using the word bank system. The clerk will inform you you can use Mystery Gift, accessed from the main menu when you boot up the game...provided you save before turning off the game!