You'll get this achievement during episode 3.
While walking through the jungle you'll stumble across the hatch, Locke will appear and say; "Looks like you've discovered one of my secrets". Now walk forward to the side of the hatch and examine the numbers, from the small menu select your camera and take a photo of them, as soon as you do the achievement will unlock.
If you're having trouble locating the numbers use the video below.
* Credit to 360VideoGuides for the video