The graphical switchboard editor is actually the same window that is used to operate the switchboard:

You can toggle between edit and operating mode by clicking the wrench icon in the top left corner of the window.

In the edit mode you see the grid and the tools window with the available rail elements. You place an element by selecting it in the tools window and clicking in the desired position in the grid. You can move elements by dragging or copy elements by dragging while the CTRL key is pressed. On the right hand side you can enter properties for the selected element such as the associated switch, signal, sensor etc.