Thanks for all of you for your help and precious support.
In fact, I am using this tuto
and this forum discussion
to add new schema. I fixed the previous issue I raised by setting the md5 password. So I refered to these two links
I checked if the procedure was successful by this command ldapsearch -xLLL -D cn=admin,cn=config -W -b cn=config cn=*host* and result was Ok.
However, I am trying now to add one object with the new added type (host for instance) and I am using webmin tool but I am getting this error: Failed to create new object : LDAP add of cn=host1, dc=test,dc=com failed : attribute 'cn' not allowed.
Please note that when I miss one required attribute webmin tells me about its name so I think that it (webmin) well recongnizes my new schema (host).