The Domain panel shows a nested tree-view listing of SNOMED CT domains and sub-domains.
Scroll up and down the listing via the scroll-bar to the right of the Domain panel, move up and down the list via your keyboard and keys, or drag your cursor onto the desired Domain name until it is highlighted.
The domain names are active - when highlighted, pressing one will change the context of the Applicable Attributes panel to those matching the domain, and also expand the Domain Details sub-panel at the base of the Domain panel.