I answered my own question! For those curious, it's just down near the bottom of the script. If you search for
http://www.google.com/calendar/event

and replace it with

http://www.google.com/calendar/hosted/yourdomain/event

where yourdomain is the domain for your Google apps, it seems to work!

Does anyone have any tips on how to modify this script so that it will be able to add events to Google Calendars that are created under Google Apps for domains?

Currently, when I try to add an event, it wants me to login to google calendar; whereas the calendar that I'm using with Google Apps is at the URL "http://www.google.com/calendar/hosted/<domain>"

Thanks for the script!</domain>