"New Plug-in" Greyed out

Hi,

When using vSphere client on an ESXi5.1 host, I can't add plugins because the "New Plug-in..." pop up is greyed out in the Plug-in Manager - see picture below.

I'd be grateful for any help in resolving this newbie issue.

This host started out as a VMware vSphere 5 Hypervisor Host, to which the following licence has been added.

Product: VMware vSphere 5 Enterprise Plus Licensed for 1 physical CPUs (unlimited cores per CPU)