To add a free license to your ESXi 5.5 installation, go to the VMware webpage and log in to your account. There you will find a „License & Download“ button, where you can register for a free license. You might also search for „Free products“ on the product webpage and click on „vSphere Hypervisor“ to get there.
Sometimes those „simple“ things are hard to find. So thank you to bring it up here!