Hi. I have used the guide at
http://www.virtualmin.com/documentation/id,combining_virtualmin_and_ldap/
to setup virtualmin + ldap and it works great. The only thing i had to change was Servers - LDAP Server - OpenLDAP Server Configuration - Access control options - Allow LDAP v2 clients ; with Aloow anonymous login with DN it couldn't login do LDAP.

Now, the thing is with Dovecot. I cant find a way to configure the /etc/dovecot-ldap.conf . Can anyone please give me a working example of this file ?
Thanks !

I'm still not as far as you are since I have the problem with virtualmin being unable to create LDAP users.
But dovecot with ldap and the dovecot-ldap.conf file is covered here http://wanderingbarque.com/howtos/mailserver/mailserver.html
I haven't tested with virtualmin, but it works with the ldap setup described in the howto you followed.