Configure your client software to talk to a new server, as usual. When asked for the server type, select 'GroupDAV' (some software may make mention of OpenGroupware.org servers, which are compatible with Citadel at the GroupDAV level). When asked for the path or URL, enter:
http://your.server.name/groupdav/
...and supply the same user name and password you use to log on to Citadel. There is no need to put your user name into the URL; Citadel knows your folder paths based on your user name.
You'll also want to point your client at the correct URL for retrieving other users' free/busy information. That URL is:
http://your.server.name/freebusy/
Special note for Kontact users: there is a known bug in some versions of Kontact in which the GroupDAV data will not be visible simply because you checked the box next to the resource. Instead, try right-clicking on the calendar or address book you want and enabling it from the context menu. (Thanks to Jon Watson for this tip.)