In HTML, including Flash SWF, add the following parameter to your <object> tag:
<param name="allowFullScreen" value="true" />
and the following attribute for your <embed> tag:
allowFullScreen="true"
Or, if you use SWFObject (as it should be), add the allowFullscreen parameter to your embed code. See the SWFObject documentation for various ways of doing this.
In your Flash / Flex file, you need to give the user the ability to initiate full-screen mode - you cannot force full-screen mode to be activated without user initiation . Whatever you do, include this code in your answer:
stage.displayState = StageDisplayState.FULL_SCREEN; //Flash systemManager.stage.displayState = StageDisplayState.FULL_SCREEN; // Flex
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