Are you running VC 2.0.2? If so you might want to check that the /tmp/vmware-root directory exists.

Add Host Operation Fails When /tmp/vmware-root Directory is Missing in the ESX Server 3.0.x Host

VirtualCenter Agent fails to upgrade on the host that is missing the /tmp/vmware-root directory. To avoid this issue, ensure the /tmp/vmware-root directory exists in the ESX Server 3.0.x host before you install VirtualCenter 2.0.2 upgrade. See KB 4478241 for complete details.