Looking at the Add-VMHost cmdlet, I don't believe you can just pass a vCenter name in the -Location argument. That argument is looking for some sort of object in the vCenter to add itself to (like a cluster/resource pool/folder/etc). I think the assumption should be that you've already connected to your vCenter before hand with a Connect-VIServer.
For instance, this is an example that I used to migrate VMs when we outgrew a large vCenter and split vCenter instances around:
Connect-VIServer $newvCenter
# Create new cluster
$cluster = New-Cluster -Location $newvCenterDatacenter -Name $origClusterName
# Add hosts to new vCenter
foreach ($vmhost in $vmhosts) { Add-VMHost -Name $vmhost.Name -Location $cluster -User root -Password <!!!!!!> -Force -Confirm:$false }
This was the immediate PowerCLI lines after disconnecting from the first vCenter. I create a new cluster in a datacenter with the old cluster name. I then take my array of hosts and add them to that cluster.
