Would you specify the error? If the connector is configured properly, both "Configured" and "Operational" should be shown as "Yes". As KC mentioned, you need to provide the credential of a domain user, username as domain\username .
If the option "Sync user data for existing users" is checked (default), it will not sync no account from the AD unless the user logon once the account will automatically created on the QS server. Uncheck this option.
Don't forgot to allocate token to your users otherwise they can only login to the QS server but access no resource.
There should have a task named as <connect_name>_usersynctask automatically created and associated with the your Active Directory connector. If there is having a problem, the task's status shows as Failed/Error, clicking the exclamation mark icon will give some cues of the issue.
You might also check the following:
1) Check the Schedulers Type (QMC Schedulers section), see if it is "Master and Slave"? Task would be failed to execute if it fails to contact the slave scheduler.
2) In connector, add LDAP filter to narrow down number of users to sync first. Not sure how many number of user you AD has and whether too many users caused a timeout issue. Might need to tune the timeout parameter.