All versions of Outlook have a default calendar folder named Calendar, and the user can optionally create sub-calendars within it.

The Windows version of Outlook allows users to also create new calendar folders. Windows users can share out both the default Calendar folder and calendar folders they created, and other authorized Windows Outlook users can access them as you would expect.

The Mac version of Outlook 2011 is only able to see the default calendar folder and sub-calendars shared out by Windows Outlook users. Outlook 2011 is NOT able to access new shared calendars from Windows users.
http://support.microsoft.com/kb/2743766