Well since you turned it in...I guess it doesn't matter. But if you wanted to know more opinions I would have went with "on their desk". An Associate could only have one desk and as SJ stated the phrase speaks to them individually. However, the phrase also references more than one associate so "desks" would work as well. This is one of those, either way it was correct. Now if your sentenced read, "that each associate can display", it would have definitely been "desk".