Play the "Media Blitz" mission. Destroy the lone red building on a small floating platform at the far right side of the map. Notice the a pathway to it from the small enemy expansion base that is found just north of your starting position. It is marked as a civilian building when hovered over, but after it gets destroyed "secret documents" will appear. Collect it with a ground unit to unlock the secret mission. Complete the current mission. The secret mission is now accessible through the Starmap. Note: You cannot play the secret mission after going to Char. You must go to the "Load" menu and load a game from before Char in order to play the secret mission.