You will have to first build/install a ESXi host. Then the OVA that you downloaded will work. you will have to deploy the OVA on the ESXi host.
Follow this video for steps to deploy vCenter
http://www.youtube.com/watch?v=YXFuQEIwNig
This is the KB Article which will help you with the same.
VMware KB: Downloading and deploying the vCenter Server Appliance 5.x