blog.tartiflop

At the time of writing, the Adobe Flex Builder 3 plugin is not compatible with Eclipse 3.4 (Ganymede). Flex Builder requires a version 3.3 of Eclipse during its installation and will report an error when you point it to a 3.4 version. However you can force it to continue with the installation and then install a patch, available with thanks to Tekool.net. For the patch to work fully you need to cleanly restart Eclipse (as mentioned in the comments on the linked post) :

eclipse -clean (on windows, or)
./eclipse -clean (on mac/linux)

This is needed only once - after this the plugin works fine with the latest version of Eclipse.