How about:

     <key>DirectoryClass</key>

<value>twistedcaldav.directory.appleopendirectory.DirectoryService</value>

     <key>DirectoryConfiguration</key>
     <value>
       <dict>
         <key>node</key>
         <value>/Search</value>
       </dict>
     </value>

   For static users:

     <key>DirectoryClass</key>
     <value>twistedcaldav.directory.static.XMLDirectoryService</value>

     <key>DirectoryConfiguration</key>
     <value>
       <dict>
         <key>filename</key>
         <value>/etc/caldavd/directory.xml</value>
       </dict>
     </value>