To add a New Domain to your DNS Manager, you need to select the "Add Domain" button on the DNS Manager Overview Page, this will then take you to the "Add Domain" page, an example of this can be seen below;
From within this page, you can enter the domain name you want to add (e.g. "demo.co.uk") and you can select the Zone to add it to, here you can either select "New Zone" (default) where this will create new DNS Zone with this domain in for you to use, or you can select any existing Zone to add this domain too.
Once you have provided the Domain and selected a Zone, press the "Add Domain" button, to add the domain to your DNS Manager. Once completed, you will be redirected to the Zone Records Page.