You use the Flex Component Kit for Flash CS3 to create Flex controls, containers, skins, and other assets in Flash CS3 Professional, and then import those assets into your Flex application. The documentation that is currently shipping with Flex Builder does not contain the information on how to create Flex containers. This article contains a documentation update with this information.
This documentation will also be included in the next beta release of Flex Builder.
Download the PDF: Importing Flash CS3 Assets into Flex