Ubuntu Pro can be enabled from the Software & Updates application.
Go to the "Ubuntu Pro" tab and click "Enable Ubuntu Pro".
On the next screen you will be presented with a six-character code as part of the first option.
Enter the code into the field above on this page, select the subscription you want to attach to, submit.
During installation, you'll be asked whether you want to Upgrade to Ubuntu Pro.
Choose "Enable Ubuntu Pro".
On the next screen you will be presented with a six-character code as part of the first option.
Enter the code into the field above on this page, select the subscription you want to attach to, submit.