Fix cake according to box instructions. Bake and let completely cool. Poke holes in the cake with a fork. Sit to side.
In a small pot, add pineapple concentrate, water and sugar. Warm, do not let boil. Remove from heat and begin to spoon or pour over holes in cake. Save some pineapple juice and sit to the side.
Once the cakes are cooled start to assemble. After you pour the pineapple mixture on the cake, begin to add a layer of cool whip on top of that. Drizzle some pineapple mix on that and do it all over again with the top layer and you're done! (Extra tip: store any leftover pineapple mix in fridge, so when you cut a slice a cake, you can drizzle extra on each slice you cut) Enjoy!