VMware cluster inside a folder cannot be found
When a VMware cluster is underneath a folder instead of directly under the datacenter, no clusters will be listed when adding the compute resource.
Instead, you'll get an error saying "cluster is required for this operation".
Moving the cluster directly under the datacenter will workaround the problem. http://projects.theforeman.org/issues/1945#note-13 describes this in a bit more detail too.