1. In Flex builder, create a new Flex project (File -> New -> Flex project).
2. Copy all the contents of the ArcGIS_Flex\samples\src folder to src folder of the Flex project created above.
3. In Flex builder, add the API Library to the project through Project -> Properties -> Flex Build Path -> Library Path -> Add SWC (and locate the "agslib-1.2-2009-05-15.swc")
4. In Flex builder, right-click on any of the sample mxml applications and click "Run Application"