Hi Juliano,
Can you post what you are using on the "source" tab?
The value that you want to pass to CN depends on what you are getting as a return value from the source. For example if on the return value the attribute "cn" holds the value that you want to pass on to CN attribute, you could use it directly. An example about how you pass on the value could be:
CN=%cn%%rep.LDAP_STARTING_POINT%
Hope that helps.
Best regards,
Tahir