Hi Flynn,
Have you installed the Supporting Add-on for Active Directory (LDAP-search) ?
Take a look in here for any error message, $SPLUNK_HOME/var/log/splunk/SA-ldapsearch.log
Add-on Docs (troubleshooting): http://docs.splunk.com/Documentation/SA-LdapSearch/2.1.2/User/UseSA-ldapsearchtotroubleshootproblems
Did you map "winfra-admin" role to your user account ?
Did you deploy "TA-DomainController-NTx" Add-on on your Domain Controller based on OS version ?
You should read this http://docs.splunk.com/Documentation/MSApp/1.2.0/MSInfra/TroubleshoottheSplunkAppforWindowsInfrastructure
Hope it helps.
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