openldap logs via syslog so the details will be in /var/log/zimbra.log. By default we only log error level messages (ldap_log_level 32768). If you want to see query level logging you will need to change ldap_log_level to 256 and restart the ldap process.
Code:
% zmlocalconfig -e ldap_log_level=256
% ldap stop
% ldap start