Use one of these solutions to restore your calendar data.
Manually copy calendar data
If you copied your calendar data manually to "Users/~/Library/Application Support/iCal" but only the default "Home" and "Work" calendars appear when you open iCal, quit all open applications (including Mail and iCal) and move the "Users/~/Library/Calendars" folder to the desktop. Open iCal again.
If you created an iCal Backup File
Double-click your iCal Backup File (this is the file that's created when choosing Backup Database from the File menu in iCal). This will restore your calendar data.
When you next open iCal, your previous calendar data should be restored.