You do have Leopard, right?
To set up CalDAV support for Google Calendar in Apple's iCal, follow these steps:
Download the set-up tool
here.
Open the tool and enter your Google Account username and password.
Click Sign in.
Select the calendars you'd like to add to iCal (Note you may only add calendars for which you have ownership privileges), and click Add to iCal.
Add your email address to your Address Book card by selecting Add Email. You'll be prompted to add your email address only if your address is not already in your Address Book.
You won't be able to invite or email guests to Google Calendar events within iCal if your address is not in your Address Book.
Your Google Calendar will now appear in iCal's list of calendars, and changes you make to your Google Calendar calendars in iCal will show up in Google Calendar.
Event information will automatically refresh every 5 minutes. You can change this interval by opening the iCal > Preferences > Accounts menu and selecting a new refresh interval under the 'Refresh calendars' drop-down. Keep in mind you can also force a refresh by selecting a calendar and pressing the Apple and R keys at the same time.
Remember, you can only add calendars that you're allowed to make changes to in Google Calendar.
Calendars that are checked but disabled have already been added to iCal - you can remove them from within iCal.