Launch iCal then choose preferences and click accounts tab.
Click the plus (+) sign at the bottom of the window. In the Add an Account sheet, choose GOOGLE from the account type and enter your Gmail/Googlemail Email and your password in the right fields then click the Create button.
iCal checks the account details and adds it to iCal, you will now see a new calendar group with your googlemail address as it’s label.
My tip here is, if you only use Google Calendar, take out the Computer calendar by using the (-) minus symbol on Preferences.
Don’t forget you can use Google Calendar on your iPhone, iPad and iPod touch to keep all in sync.