In the 2nd DLC level "Crate of Secrets", shortly after the level starts, you will pull a lever. This will cause a rock to fall and smash the crate and the achievement will unlock. You cannot miss it if you finish the level.
NOTE: All the levels in this DLC are already unlocked. You can just play the required level for the achievements if that is what you want but I recommend playing all the levels because they are fun and easy and to get the most out of you $2.