Click 'SEO' in the left sidebar navigation > Click New Domain button.

If your account is new or does not have any domains tracked yet, you will be presented with welcome screen upon login. If chosen SEO, you’ll see a new client page that allows to add your client and later you’ll see a box to add your first domain to be audited.

For those who already have at least one domain in their account, you can add additional domains by clicking the SEO in left sidebar navigation and “New Domain” button in the “Domains List Page.”