How to Use Tween in an ActionScript Project (Flex Builder 2)
From moockbog.org: http://www.moock.org/blog/archives/000197.html
To use the Tween class in an ActionScript Project in Flex Builder 2 you must include all dependent class libraries in the application's library path. Specifically, you must include the Flex framework's framework.swc and the appropriate version of framework_rb.swc, depending on the current language.
