You will drag and drop a new table definition onto it, from the Component Palette.
Each offline database object that you create in a project must exist as part of an offline database definition. It is possible to model objects from multiple schemas and from databases of one or more different types (Oracle, MySQL etc.) in one project. A database diagram can be used to copy online database objects to a project and create offline definitions, or it can be used to create new offline database objects that can later be generated into an online database.
As you add database objects to an empty diagram, you will have to specify the location and choose or create an offline database definition for the diagram.
Although each diagram is associated with one offline database definition, you can display objects from other offline database definitions on that same diagram.