Sorry. I assumed you are running the latest version, i.e., 5.2. Mix scenes were introduced with v5.0. If you're running 5.0 or higher you should see the mix scenes icon by default. If you're running 5.0 or higher and don't see the icon it might be due to something in your S1 preference settings - check the manual. If you're running anything prior to v5.0 you don't have mix scene capability.