Click three or more times in the left window to create a profile shape. Try to make the first and last points as close to the dotted line as possible. When you're done, click 'Lathe' and see your creation turned into a 3D shape!
You can rotate the 3D shape by click and dragging your mouse over the right window, or export to STL format for 3D printing. Running through netfabb before attempting to print is highly recommended.