Flash CS4 ActionScript 3.0 Tutorial
How to Assign Alpha Property to Button Symbols, Movie Symbols and Text with ActionScript in Flash
The Alpha property controls an object's transparency. Acceptable values range from zero to one, with zero being transparent and one appearing opaque.
Movie Symbol with instance name alphaMovie_mc
Add a Movie Symbol to the stage.
Select the object and open the Properties panel (Cmd/F3 (PC-Ctrl/F3)). Type in the name alphaMovie_mc for the Instance Name.
Add a new layer called Actions.
Option double-click (PC - Alt double-click) the keyframe in the Actions layer to open the ActionScript panel.
Copy/Paste the following code to the ActionScript panel.
alphaMovie_mc.alpha = .5;
Test your movie by pressing Cmd/Return (PC - Ctrl/Enter). By setting alpha equal to .5, the object's transparency is set to 50%.