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.
Go for a free version!
The component is called „VMware vSphere Hypervisor 5 License“. Just enter your data and proceed and a license will be shown to your back on the web page.
Here is your license key of ESXi 5.5
After that, go to „Configuration“ > „Licensed Functions“ in your vSphere Client.
Go to Options
Open the options in the upper right corner.
Add the license key
When you save your key, your free ESXi 5.5 license will be enabled.
Sometimes those „simple“ things are hard to find. So thank you to bring it up here!