Yeah, read my page on it, it explains it all.
Other suggestion, realize that most LDAP documentation is as bad as most Linux docs.
Seriously. inetorgperson schema should be included already Look at the lines up at the top of /etc/openldap/slapd.conf. Isn't include inetorgperson schema already there without a # (comment sign) in front of it? If not, uncomment.
Now, mail is an attribute of inetorgperson. (By the way, it's schema, not scheme. Be careful of typos---you'd be amazed how much time you can waste on ldap with typos. Trust me on this.)
So, if you want user1 to have an email address, you'll use
dn: cn=user1,ou=domain,dc=example, dc=com
You don't need ObjectClass: person, because, as it's a SUP (parent) of inetOrgPerson, (as you can probably figure out if you delve through the schema), it's implicitly already there. (As is objectclass: top).
My page is at http://home.roadrunner.com/~computertaijutsu/ldap.html
It has a link to a schema browser, at akbk.home (or something similar---ahh, http://ldap.akbkhome.com/index.php
There you can find objectclasses with lists of their attributes.