Managing licensing keys in vCenter
This video from VMware Kb blog is showing that managing licensing keys in vSphere is made much more easier than it was the case in ESX 3. Basically what you need to do to manage your licenses is that you need to access with your Virtual Infrastructure client (VI client) your vCenter Server. You can't manage you licensing by pointing the VI client directly to the ESXi 4 or ESX 4 server. This won't work.
You can use the VI client installed on your management station or your VI client installed on the vCenter Server which you probably access via RDP connection. See some good freeware to manage all your RDP (but not only) connections.
Once connected to the vCenter just click Home > Licensing > Manage vSphere licenses.
Then there is a wizard which helps you add or remove licenses. Follow the detailed step by step procedure on the VMware KB website.