If are trying to embed button symbols (extended SimpleButton) from Flash CS and the results are not what expected:
[Embed(source="assets/elements.swf", symbol="my_button")] public static const BUTTON_CLASS:Class; ... addChild(new BUTTON_CLASS())
try exporting your elements into .swc and attach into Flex project, later in code do:
public static const BUTTON_CLASS:Class = my_button; ... addChild(new BUTTON_CLASS())
This may also help you capturing Event-s dispatched on custom frame from loaded MovieClips.