Why won't my filters apply in Final Cut Express?

Hi! I have been asked to lighten a short video clip downloaded as a MOV file from YouTube. I have it in Final Cut Express and drag over the “Color Corrector” video filter. When I make changes in the video filter settings, I don’t see anything happen in the viewer. Is the filter working? Am I missing a step or two? I’m new at this if you can’t tell. Thanks!