After you have registered the required database(s), the corresponding alias(es) appear in the DB Explorer tree on the left. If the Show Hosts option is checked on the Environment Options | DB Explorer page, the host nodes are also displayed in the tree (alternatively, you can use the Show Hosts item of the Database context menu, or the drop-down menu of the View Mode toolbar button for the same purpose).
DB Explorer displays all registered databases. Connected/disconnected databases can be easily distinguished in the tree: aliases of disconnected databases are grayed out.
To connect to a database, simply double-click its alias (or select the database alias in DB Explorer and press Enter). If the connection is successful, the database node expands into a tree of objects.
Now you can navigate within the database objects. Use SQL Assistant to get extended information about the currently selected object.