1. You may list Domain user from Domain Controller command line using
When Linking Connect MC to LDAP server you need to paste username as the output from dsquery.
2. After you successfully linked MC with LDAP server, you'll be able to import AD user and groups using blue '+ ADD LDAP USER OR GROUP' button
3. If you'd like to add Security Group to MC, bear in mind it must be of global type which can be checked from Active Directory Users and Computers windows at Domain Controller.