Easiest way to describe is to go along the concourse until you see the Casino and Mercure Hotel, and then go upstairs to the upper level. This is near where the KLM and BA lounges are, and if you walk around a bit you'll see a couple areas with lounge chairs filled with weary travelers. There's a few bathrooms up here also, and its hard to describe which one has the showers because the area is so non-descript... but check them out. One of them (that looks like all the rests) actually has two small narrow shower stalls.

If you can find the lounge chairs in the darker area that occasionally is showing images on the wall, looking at the "screen" you should go right, and its the next bathroom in the center area you come across. There's no sign for the showers, and you won't know they exist until you go inside the bathroom and look to the right for the two doors.

Warm water, good pressure, clean... they are a blessing when needed! ^