Applications built with Business Analyst Online API for Flex 2.x require ArcGIS Server API for Flex 2.x. Business Analyst Online API for Flex 2.1 was built on top of ArcGIS Server API for Flex 2.3.1.
The only currently supported SDK version is Adobe Flex SDK 4.0. For detailed Flex framework system requirements, read the most up-to-date Flex SDK system requirements by Adobe.
All applications compiled with Flex SDK 4 require the use of Flash Player 10 . For detailed flash player system requirements, read the most up-to-date Flash Player system requirements by Adobe.
The most popular Flex development environment is Adobe Flash Builder 4. It is not free, though a 60-day evaluation version is available.