This was a little frustrating. The ESXi host keeps nagging me to enter a license key - even though the hypervisor was free.
After some messing around I found it was actually quite easy:
1) Get your free license key from VMWare (you are given this duirng the download process)
2) Download the vSphere Client from the IP address of your host
3) Open your host in the left window
4) Select the "Configuration" tab
5) On the left hand side there is a "Hardware" section and a "Software" section. Look in the "Software" section and select "Licensed Features"
6) On the right hand side of the screen look for and click the "Edit" button. A pop up window will be displayed. The screen is divided into two sections and each is selectable with a radio button. Select the "Assign a new license key to this host" radio button
7) Now, click the "Enter Key..." button and you can enter the license key provided by VMWare that you got in step 1. Click "OK"
8) Click "OK" again to close the pop up.
Thats it, finished.