This achievement is earned in mission 7 "A Crack in the Slab." After getting the time turner device (one of a number of Harry Potter-like things that show up in this chapter), you need to head out of the piano room and straight across the lobby to the dining room on the far side.
You'll need to master the switching between past/present to get through barriers, etc. For example, switching to the present to move up to the barrier keeping you in the piano room, then switching to the past to find yourself past the locked doors and with the barrier gone.
Once you've snuck into the dining room (you can do this either from the first floor, or by going upstairs and working your way onto the balcony surrounding the dining room), make sure you're in the present time line, then go over to the table and find a spot where you can crawl under it. Switch to the past and you should be able to grab the key off of a cushion to unlock the achievement. Remember, you cannot knock out any of the guards in this room or alert them for the achievement to unlock.