Trying to compile a library project requiring Flex 4.0+ (or Hero) SDK in FlexBuilder 3 should be straightforward: just point Flex to the new SDK. However, like many things Flex, this simple task yields an error:
“unable to open ‘/Applications/Adobe Flash Builder Burrito/sdks/4.5.0/frameworks/libs/player/9.0”
The key is to add a compiler argument like: “-target-player=10.1". For library projects, we have to set a compiler argument since the Flex GUI does not allow for specifying flash player versions.
Check out this post for the full directions: http://opensource.adobe.com/wiki/display/flexsdk/Targeting+Flash+Player+10