You can login at this URL in order to create calendars and manage access rights for your calendars.
This server has no client functionality, i.e., you can’t read your calendars or add entries to them. For this you need a calendar application such as iCal (MacOS) or Lightning (a thunderbird/icedove extension. See the Client Configuration page for details.
Example: add an calendar to thunderbird/lightning right click in calendar list -> new calendar -> network -> Caldav -> enter URL
The URL should look like http://calendar.math.uni-leipzig.de/caldav.php/hellmund/lehre.