Press enter during gameplay to bring up a message prompt. Type in the following code to get the desired effect. Warning: These codes disable Achievements.

Complete the Campaign.

Left click any creature several times to make it explode.

Keep left clicking units to hear secret dialogues.

When in the Starcraft 2 menus press the blue symbol under the title. This will place you in a public chat channel called cow. Every time you speak your message will be followed by ...moo!

To unlock the secret mission, on the "Media Blitz" mission, where you get to control the Odin in the city, head down to the far bottom right corner of the map. There's a civilian building on a little floating platform. Destroy it, and "secret documents" will be revealed. Pick these up with any ground unit, and then complete the mission as normal. After, you will now have the secret mission available for selection from the Star Map.