Where to download vCenter

Hi Experts,

I currently have one host licensed for VMware vSphere 5 Essentials Plus (soon to grow to add another host). I am trying to find the link to download vCenter for Essentials and setting up a Windows server for it.  Where can I get the link to download vCenter for Essentials? I have the license key for it but don't see the download link for vCenter Essentials

Also we have another vCenter that is 5.1 and we want to keep the new 5.5 environment separate. Are there any gotchas when installing vCenter 5.5 with separate vCenter 5.1 in our environment? We are planning to do a "Simple install" for vCenter 5.5 on the Windows server. The hosts running 5.1 will not be made part of the new 5.5 environment

TIA