1. Go to Design > Themes and click "Edit This Widget" on top the menu widget. (You may prefer to find this by clicking the "sections" tab from the left menu, then clicking to section 2. Then add new widget
2. Go to Menu Item Type: and select Custom URL from drop menu.
3. Enter Menu Title and paste URL into the URL field.
4. Click Add Item.
5. You can drag the items in the top menu to arrange the order or edit the Title/Link as shown in screen shot below: