You may want to go to Addressbook -> Extras -> Settings and end up on the settings page Addressing (german: Adressieren below Verfassen).
Check your LDAP Directory Servers and add an entry:
Name: Zentyal Users
Serveraddress: [YOUR_OWN_SERVER_ADD/IP]
BaseDN: CN=Users,DC=[YOUR_DOMAIN],DC=[YOUR_TLD]
Port: 389
BindDN: CN=[YOUR Prename] [YOUR Lastname],CN=Users,DC=[YOUR_DOMAIN],DC=[YOUR_TLD]
OK
Remark: [YOUR Prename] [YOUR Lastname] is shown in the Zentyal config editor -> Users and Computers -> Manage, see the shown name (german: Angezeigter Name)