Select the 'Media Blitz' mission and destroy the lone red building situated on a small floating platform at the far right side of the map. Their is a pathway to this building from the small enemy expansion base that is found just north of your starting position which is marked as a civilian building when hovered over. When this building gets destroyed 'secret documents' will appear. Collect them with a ground unit to unlock the secret mission which will become accessible through the Starmap when you have completed the current mission. You will not be able to 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.
Video showing you how to get the Secret Mission in 'Media Blitz'
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