There are a few times that a system on our domain doesn't show up in either the Clients tab or the Remote Install -> Computers tab. The way I've been handling it is manually installing the client and then importing the settings XML file. I haven't looked into having the RAC communicate directly with Active Directory yet which may be one fix. It would probably be beneficial to have separate OU's in AD representing departments which would then trickle down to RAC. I'm new to the administration so I wanted to see what others were doing.