In order to combine flex and flash you have to install a flex component kit http://www.adobe.com/cfusion/exchange/index.cfm?event=extensionDetail&extid=1273018
inside flash cs3.
then you can convert movieclip in a flexcomponent and export it as swc.
When you have swc you can import it into flex project and works with movieclip.
If you want, you can see this tutorial: Combine Flash and Flex
Instead if you want only load and swf inside flex gui you have to use the swf loader component!