When you start SiteKiosk Android from the application menu of your Android device, it will ask you what to use as your default home screen. To fully enable the protected kiosk mode on your tablet you need to use the SiteKiosk Browser as your default home screen.
You also need to tick the box to use the SiteKiosk Browser as the default option for this action. This will ensure that SiteKiosk will automatically start again if the device is restarted or powered off from within a running SiteKiosk Android browser.

Note that the dialog about what app to use as your default home screen will not show again, until you reset the behaviour, e.g. by using the reset button in the Android app settings.