- Make a backup of iCal in case things go wrong.
- Add the email address Growl to your personal Address Book entry. (Mark it as secret if you have that feature enabled.)
- Copy the compiled Mail.scpt file [Hints mirror, original source] into the iCal Resources folder. You get there by navigating to /Applications -> iCal.app -> Control-click and choose Show Package Contents -> Contents -> Resources.
- Copy Mail.scpt into the iCalHelper Resources folder: /Applications -> iCal.app -> Control-click and choose Show Package Contents -> Contents -> Resources -> iCal Helper.app -> Control-click and choose Show Package Contents -> Contents -> Resources.
[robg adds: I haven't tested this one.]

