If the one you want is not visible, click on the one that is, and a pop-up menu will appear. This tool is supposed to simulate finger painting.
Color is displaced, or 'smudged' from the place where the pointer is clicked.
It is moved in the direction the cursor is dragged. How far the initial color is moved depends on the setting in the Pressure box in the tool's options bar. When editing an image with any tool that uses brushes, you can right click on the image and the brushes pop-up palette will appear right next to your cursor. You can cycle through the blur, sharpen, and smudge tools by holding down the Shift key while pressing the shortcut letter. The illustration below is the smudge tool's options palette.