cPanel is an interface used by some hosting providers such as HostGator, JustHost, and WebhostingPad. If your provider uses cPanel, these directions will help you set up your CNAME.
How to add a CNAME with a cPanel control panel:
Log in to your account with your particular hosting provider.
Find the window entitled 'Domains.'
Click on 'Simple DNS Zone Editor' - This will take you to a control panel.
In the control panel you will see a section titled 'Add a CNAME Record':
In the 'Name' field of this section, type the name of your chosen subdomain. For example, if you picked promos.mydomain.com as your custom domain, enter promos here. Or, if you're using a root domain such as www.mydomain.com then you'll want to enter www here. cPanel automatically fills in the rest of your URL.
In the 'CNAME' field, type unbouncepages.com.
Click the 'Add CNAME Record' button at the bottom.
You can now return to your unbounce account and set up your custom subdomain by going to your dashboard and clicking account>custom subdomains and following the steps.
Keep in mind that these kinds of changes take a certain amount of time to propagate across the internet. You may have to wait up to 24 hours before you see your changes take effect.