stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);

			
private function onKeyDown(event:KeyboardEvent):void
{
	if (event.keyCode == Keyboard.BACK) {
		event.preventDefault();
		log.appendText("Back Pressed\n");
	} 
	else if (event.keyCode == Keyboard.MENU) {
		event.preventDefault();
		log.appendText("Menu Pressed\n");
	} 
	else if (event.keyCode == Keyboard.SEARCH) {
		event.preventDefault();
		log.appendText("Search Pressed\n");
	}
}

As an added bonus you also have access to the keys on your device. Most devices have an HOME, MENU and SEARCH button on the device itself. Of course you can access them. Add an normal key down listener to the stage and respond to it.

Accessing System keys in AIR for Android

ActionScript 3