on some earlier versions of OSX 10.6.8 there might occour an error, preventing the mac internal installer to execute the plugin install process correctly. This is an issue exclusively on OSX 10.6.8, versions above are not affected. If you encounter this problem, please download the hotfix below provided by apple:
http://support.apple.com/kb/DL1512
the apple homepage wrote:About Apple Software Installer Update 1.0
This update addresses an issue that may prevent certain Apple software from installing. This update is recommended for all users running Mac OS X 10.6.8.