Decide which appointments you want to use and export them from an other calendar client or a website.
Then you have to click on the down-arrow button of the calendar list.
Afterwards, you browse and choose the iCalendar or CSV file.
Finally, select the google calendar where you want to import them and click on import.
Source:
http://www.google.com/support/calendar/bin/answer.py?hl=en&answer=37118