I figured i would add this here since there is a lot of misinformation on this one.
On chapter 4. go through it normally till you get to the giant sauna, as you leave that area you will come across a stairwell with windows on each floor. the first window you can see a slide inside, the second window you will either see this duck, or nothing. the third window you will see some furniture. if you saw the duck, zoom in on him (mouse wheel) and get the achievement. if not, you DO NOT NEED TO RESTART LEVEL like everyone says. just keep going down the stairwell then go through the tunnel archway. head into the water and to the next tunnel archway, stop just inside the tunnel archway, turn around, go back to the window. repeat till the duck appears. this has been extensively tested by me and 1 other person and confirmed to work. its way faster than restarting the map.