Hi Chris,
When you do a fresh installation usually it creates those relevant user on its own. In case you want to continue from the same phase where you are stuck then you can create the user <SID>ADM and then come back to SAP installation screen and click OK to proceed.
I guess you may be using Windows Server 2008 then do a right click in My Computer > MANAGE then follow below screen to create the user:
Regards,
SUJIT