You have to configure your LDAP Server in nedi.conf like that:
#ldapsrv ldaps://host.domain 636 PASSWORD DC=XX,DC=YYY USER uid
ldapsrv ldap://your.server.net 389 bindpassword OU=People,,DC=domain,DC=dom your binddn like (cn=ldapadm,ou=services,dc=domain,dc=dom= uid
After that you can see a new Button at the Account Config Site near the "Add" Button. It named lile "Add LDAP User" in German: "aus ldap hinzufügen"
Now you type in a valid Ldap Username and click the add "ldap user button".
That's it.