Yeah, a lot of applicants have a hard time with this one. When we think of "ethics" we tend to immediately think of a big Enron-type scandal.
For most of us (including me, when I applied to b-school), the situation that will apply is usually much more subtle. The most interesting stories aren't necessarily "big" stories (e.g., uncovering billion-dollar fraud) but the ones that aren't black and white. Some of the best essays that I've reviewed in this area have to do with seemingly small issues, but where the applicant chose to do the right thing, instead of taking the easy route.
Just as an example, imagine you uncovered a mistake in a presentation at the 11th hour. The easy thing to do might be to just let it slide, since no one would probably notice, anyway. But you decided to alert the team, take the blame, and work late to make sure that it got fixed because you didn't want to let the team down. Seems mundane, but that's still a valid story that illistrates the point.
I hope that helps!
Scott