From the documentation:
Use your favorite editor to edit the provided slapd.conf(5) example (usually installed as /usr/local/etc/openldap/slapd.conf) to contain a BDB database definition of the form:
database bdb suffix "dc=<MY-DOMAIN>,dc=<COM>" rootdn "cn=Manager,dc=<MY-DOMAIN>,dc=<COM>" rootpw secret directory /usr/local/var/openldap-data
But, my domain is either 127.0.1.1 or localhost,
What should I put instead of MY-DOMAIN and in COM ?
You can put whatever you want. Suffix has nothing to do with what IPs/interfaces slapd listens on. Suffix is the top level of the data tree, it doesn't have to be DNS/network names. Think of it as your database name. So while you could use:
suffix "dc=localhost,dc=localdomain"
or
suffix "dc=example,dc=com"
you could also use:
suffix "dc=my,dc=ldap"
You can have more than two parts if you want
suffix "dc=my,dc=ldap,dc=server"