Flex development
http://resources.arcgis.com/en/help/flex-viewer/concepts/
Developing for ArcGIS Viewer for Flex
ArcGIS Viewer for Flex was written in Flex 4, using the most current version of ArcGIS API for Flex. To edit Flex source files, you need to know about Adobe Flex.
As a Flex developer, you can use Flash Builder to modify existing widgets, create new widgets, or rip it all apart and use it as a starting point for your Flex application. The most common integrated development enviroment (IDE) for Flex development is Adobe Flash Builder 4.5, but you should also be able to use other IDEs that support Flex 4 development.
To learn more about the underlying Flex, see ArcGIS API for Flex.
For a visual introduction, see the video, Configuring and Customizing the ArcGIS Viewer for Flex, which is a Technical Workshop from the 2011 Developer Summit.
For help to get started, see Getting Started - Developers.