First, you have to change one of your web part zones to be a widget zone. Then your users or editors if you choose to do so can add widgets. To change a web part zone into a widget zone, click on the "down arrow" on the left of a zone. Choose properties. At the top you can choose how you want to configure this as a widget zone. By default it will have None for widget zone type. Here you can choose "User Personalization" or if you want only content authors to use, select Customization by page editor. Now users can go through and add content via the big green plus signs to content.