To use OpenLDAP authentication, you should first create an admin user with the same username as you plan to use when you switch to LDAP so that your LDAP user will already be in the admins role when you fisrt are able to login using LDAP.
In site Settings:
Log in as the admin user with the same username as your LDAP user.
Go to site settings click Use Ldap Auth and specify Ldap Server, Ldap Port, Root DN for users (in the form "cn=admin,dc=example,dc=org"), and Admin Email. You will have to enter your password to test the authentication can work before it will alow you to save with Use LDAP = true.
Check the box for Automatically Add LDAP users on first login.