Since it bit me a couple of days ago: The AppleScripts to be used as Folder Actions must be stored in ~/Library/Scripts/Folder Action Scripts or /Library/Scripts/Folder Action Scripts.
The former is the location for Scripts available to the user of the account, the latter for all users of a machine. Don’t get fooled by the fact that you are able to choose any script via the context-menu of a folder if Folder Actions are enabled.
Just in case you need to know. 