add flex combobox to movieclip in swf

Question

I am trying to add a flex custom component (basically a combobox that is populated with xml) to a movieclip in a swf. Is this possible? I don't get any errors with the code below, but I also don't see the combobox. If I add it to the parent, which is a uicomponent, then it shows up.

_fontDD = new FontDropDown();       
_addTextMenu.addChild(_fontDD);

_addTextMenu is basically a movieclip symbol within a swf that I am bringing in as a swc.