ESXi 5.5 Host and Adding to vCenter 6.0

I have installed vCenter 6.0 and when I create a data-center  and then try to add host I get license errors. The ESXi 5.5 hosts are already licensed. I have vCenter 6.0 Essentials which allows for 3 hosts. How can I add the existing host and I was told by VMware that vCenter 6.0 can manage ESXi 5.5 hosts.