Joining the vCenter Server Appliance to an Active Directory Domain in vSphere 6 is much different from in previous versions. Previously, you would use the Virtual Appliance Management Interface (VAMI) by navigating to the FQDN of the server over port 5480. In vSphere 6.0, you need to perform the steps through the vSphere Web Client. Once you know the new process, the task is relatively easy and quick.
Note: You can only join a PSC or VCSA with an embedded PSC to an AD domain.
Step 1. Navigate to the vSphere Client Web Client https://FQDN/vsphere-client
Step 2. Select Administration
Note: One interesting thing I noticed is that the domain never populated in the field until after I rebooted the server.
Step 6. Reboot the VCSA server.