Last week when I would like to activate the VMware ESXi 5 license through vSphere client, I was confused because I didn't know where the menu location for entering the license key.
I found it after take a long time look for.
To enter the license key for VMware ESX 5 using Vsphere client :
Select ESX host
Go to Configuration tab
Inside Software section, choose Licensed Features
At the right of page you will see the Edit..., click it
Click Enter Key..
Write or copy-paste the license key to the column provided
You will see the Product, CPU capacity and expire time according to the license key you entered.