To keep your control panel as tidy as possible, UberMenu Widget Areas are not enabled by default. To enable them, navigate to Appearance > UberMenu and enter an integer greater than zero in the Number of Widget Areas box. Save the options and you will see UberMenu Widget Areas appear in the Widgets control panel.

Please review the Adding Widgets to UberMenu instructions.

Please note that adding widgets to top level menu items is a separate process, since this is almost never what is really needed: Top-Level Widgets