Instructions
Launch "StarCraft II: Wings of Liberty" and start a single-player campaign that has access to the "Media Blitz" mission, but has not yet progressed to the Char planet.
Play the "Media Blitz" level on any difficulty. The mission will start with you controlling the Thor war machine with the objective to destroy as much of the three Dominion bases as possible before the timer runs out.
Direct the Thor to the south-east corner of the map and fight your way through the Dominion base located there.
Find a road near the communication tower that leads off to the south-east, and follow it until you find a strange Dominion science building.
Use the Thor's area-of-effect attack to destroy the building. An item called "Secret Documents" will drop from the ruins and can be picked up by the Thor. As soon as the item is acquired, the secret mission will become available from the Hyperion map.