When you first load SQLEditor you will typically see an empty new document window, the inspector window and the object palette.
One of the first things to do when designing a database is to identify the tables you need and add them to the database.
There are usually several ways to do things in SQLEditor and you can use whichever seems easiest. To create a table you can click the add table toolbar button, drag the table icon from the palette or choose the add table item from the menu bar.
To add a to the table you need to click on the new table to select it. When you click on an object the Inspector panel will change to show details of the object. When you click on a table the Add Field menu item will become available and you add a field. You can also drag a field from the palette.