Note that this will automatically select only objects of class user, but will generate warnings for non-user objects (e.g. group objects). To avoid those warnings, create a filter that combines the above example with the one in example 1, with a logical AND:
When testing and trying to find the right filter, you may have to repeatedly create, test and delete your filter. It may be easier to use an LDAP browser like the one at http://www.ldapadministrator.com/download.htm. This will connect to your LDAP server and offers a search feature where you can enter the Base DN and Filter in the same syntax as in DMM (Note: the download page offers both the free Browser tool and the trial version of Administrator - the Browser tool is sufficient to test your filters).