On the second level of Episode IV, called "Duke Burger", play through the level until you get to the bathroom where you shrink yourself, go through the vent in the right corner. Then you will be in the kitchen, turn right and go into a hallway with a red key door. Go through the red key door and the poop is at the side of the treadmill thing after the laser trip bombs. Step in it to receive this achievement.
You can also get it in a Dukematch on the same level and also on the level "Lunar Reactor". In the area with the first key card, there is a portable med kit and it is in here, just walk around. It is on the way to the key card, you can't miss it.