This tip I’m stealing from Amy over on the flexcoders list.
One not so well documented feature of Flex Builder is the use of the libs directory. By default, when you create a new Flex Project, a libs directory is created. So say for example, you want to create a project, and you are planning to use the Cairngorm framework.
All you need to do is copy Cairngorm.swc into your libs directory, and you’re good to go.