I use the Softerra LDAP browser:
http://www.ldapadministrator.com/download/index.php
the user dn I auth with is uid=zimbra,cn=admins,cn=zimbra
and I do not specify a base dn. interesting places to bind are the root, cn=zimbra or cn=com (or replace com with the top level domain you have).
Binding to root shows everything
Binding to cn=zimbra shows server stuff
Binding to cn=com shows your users, dl's, etc