Your vSphere 5.1.x license enables the vShield Endpoint 5.1.x functionality and no additional vShield Endpoint license is required.
You must download and install the vCloud Networking and Security 5.1.x binary.
You can download the latest software, available at VMware Downloads.
Notes:
If you are exclusively using the vShield Endpoint feature of vCloud Networking and Security (vCNS) in vCenter 5.1, the vCNS asset may show up as unlicensed or with an evaluation license in the vCenter Server console. Even in evaluation or unlicensed mode, the vShield Endpoint feature will continue to be fully functional.