There are two ways to obtain this.
1) Start the game up, and then wait a week before playing again. Try at least 8 days just to be safe.
2) If you just want to get this without the hassle of time passing, cut the connection to your Xbox 360 so you can change your console date. Put it back a week from your current time, and then when prompted for a storage device, choose whichever doesn't contain your original save to create a new one. Reconnect your Xbox 360 online to return to the current time, load up the save file you had made prior and it should unlock on the game menu selection.