in vCenter 5.1 there are two types of keys ... vCloud Networking and Security (which ever version you have) OR you can use vCloud Suite (which ever version you have).
When you register vCloud Networking and Security to vCenter, it should appear as an asset. There was some issues with the 5.1.2 appliance of vCloud Networking and Security.
Can you go to the download page of Networking and Security, and download the 5.1.2a maintenance bundle and apply it? Afterwards, register it with vCenter one more time using the same information as before.
see if that helps.