It sounds as though you haven't set the path in your Global Options. You must set the Flex SDK globally ( see http://www.sapphiresteel.com/products/amethyst-ide/amethyst-help/article/amethyst-global-options
). If you only have the Flex 4 SDK installed, you must set the path to that and make sure that and make sure that you only create Flex 4 projects. Can you let me know which path you have set in these global options? Is it set to your Flex 4 SDK path and do you still have problems when building a newly created Flex 4 project?