Essentials as well as Essentials Plus is only sold in Kits, where each Kit includes one license for vCenter Server for Essentials as well as 6 CPU licenses (for up to 3 hosts with 2 CPUs each. Btw. if you do have a current subscription or if the subscription was valid at the time vSphere 5 was released you may be able to upgrade your licenses free of charge in your My VMware portal.
André