How to add an ESXi 4.1 in a vCenter 5.5?

Hi

We are involved in a migration project since ESX/ESXi 4.1 to ESXi 5.  Now all my infrastructure is managed by a vcenter 4.1 and it manages several ESX 4.1 and ESXi 4.1.  Recently we've bought a new license for vcenter 5.5 and I've installed it.  Before upgrade the hosts to 5 release, I want to manage them into the new vcenter but when I try to add them into it, I get this error message:  "The host is licensed with vsphere 4 Hypervisor.  The license edition of vcenter server does not support vsphere 4 hypervisor."

How can I get this license key?

Thanks for your help

Best Regards!