When you start you will see this screen. Click the small cog icon in the top left.
You will be presented with this screen. You need to click the “System Settings” icon
On the Left of the screen highlight ADDONS
On the Right of the screen you will see “Unknown Sources” To the Right of that you will see a slider On/Off
Slide it till it is White.
When you do that you will get a warning screen Click Yes Now the Slider should change to White.
That’s it. Now you can go ahead and install our, or any other 3rd party Apps.
Go back to the Home Screen in Kodi 17 and you will start with this
On the left hand side, as indicated, click Add-Ons
Click the top left icon “System”