The base DN is string which you should be using to search any user in LDAP server. You need certainly the proper credentials to login into it but to query for any user, normally you should be using the string like "CN=<user_name>,O=<myDominoOrg>". If you provide this string in LDAP search dialog box or any other search option (depending on LDAP browser used), you should be able to get all the attributes of the user.
For example, For my very simple test deployment, I have org named 'test_zimbra' and one of the user in it is 'test_user1' then the base DN string to find this user would be "CN=test_user1,O=test_zimbra".
Tool requires the admin user DN path, so if admin user is name 'Administrator' in my setup then, i will provide 'CN=Administrator,O=test_zimbra' in tool.
Hope this helps.