Can't get symbol to flash...

I feel this should be simple to do, but I have yet to be able to get it to work.  Simply put, I need the light red box over "Pulp Mill Sources" to flash, or blink.  I have tried using keyframes and changing the alpha level at various points, but the level never seems to actually change.  What is the easiest way to do this?  By the way, the Block Diagram is a Bitmap that I have loaded to the stage, and am working on.