I beat the campaign and then saw that I was 25/26 on missions. I did some research and found out about the secret level, Piercing the Shroud. I went back and got the secret (avoiding a spoiler here) and then beat the level. However, on my "Mission Archives" screen I can't find that level. Also, I can't get back to the Star Map to select it. What do I have to do to select that level, or do I have to start the campaign over?
Uh...all these other guys are way off. Maybe this was fixed somewhere along the way? At least in whatever the current version of Starcraft II is, after you finish the campaign, you can go on back to Media Blitz from the mission screen, and pick up the secret papers. Then after you finish the Media Blitz mission again, the Piercing the Shroud mission should appear at the very bottom of the list-after the All In Char mission. There's not a lot of plot development in this situation, though...it just sends you back to the mission screen after you beat it.
And yes, if you beat Piercing the Shroud like this it does count towards the completionist achievement...