Depending on how you want to use your calendars, there is a very simple way to setup the synchronization between iCal and Google Calendar.

Easy Setup: (Using iCal v.4.x)

1. Open iCal
2. Click on "Preferences"
3. Click on "Accounts"
4. Click on "+" to add a new account
5. Under Account Type, Select "Google"
6. Enter your Google Apps Account information (Full e-mail address and password)
7. Click on "Create"
8. The system will configure/authorize the calendar and you will be synced!

In order to see other calendars from other accounts, be sure they are being shared with your web version calendar first, then you should be able to see alternate calendars in your iCal.