How to see video fx adjustments in real time?

I'm sure this is a stupid question but I searched this forum and I looked in Vegas Help with no success. All I want to do is adjust the brightness of a media clip in Vegas Pro 11. I see how to right-click the clip in the timeline and select Media FX. I can then select Sony Brightness and Contrast and that gives me a window to make adjustments. The problem is that when I slide the adjustment, I cannot see the results of my adjustment. I can't tell how much brightness I need to add. In the video preview and in the trimmer window, only the original video is displayed. It would seem that my brighness adjustments are having no effect. When I render the video, I do see the change in brightness, but I made too much change or too little change. Then I have to go back and try again and re-render. Over and over. I need to see the change in real time as I slide the adjuster. How can I see it real time? What am I doing wrong?