To use an LDAP server for Enterprise Server security, or to hold Enterprise Server configuration data (also known as an MFDS repository), you must have an LDAP client library installed.
Some operating systems include one with the default installation, or as an optional package. Microsoft Windows includes Microsoft's LDAP client as part of the base OS. AIX ships with IBM Directory Server, an installable option, which includes an LDAP client. Solaris also includes an LDAP client which is usually part of the default installation. OpenLDAP packages are available for Red Hat and SUSE Linux distributions and can be installed with rpm or yum respectively.
You can also obtain the OpenLDAP sources from http://www.openldap.org/ and build the OpenLDAP client library by following the instructions there.