Make sure you have saved the MailFOO script, somewhere you will not delete it, i.e. don't leave it on the desktop!
I would recommend putting it in your users Applications\Applescript\ folder.
Once you have the script in place, start iCal and create a new event by double clicking on a day you want MailFOO to run.
Double click the New event to bring up the options for it and select edit.
Set the name to MailfOO.
Make it an all day event.
Set it to repeat every month and to never end.
Set the alarm to Open file, and then select the option just below (normally iCal).
In that option click on the Other option and then browse to wherever you put the MailFOO app and select it.
Tell it to run the same day, and set it to a time that your Mac is likely to be on.
Job Done
It should look like the screen grab below.
That should be it.
Click here to go to the download page.