Kodi 17 Krypton has changed from the old interface to a fresh new look.
It also has changed some options including now needing to Enable Unknown Sources to install add-ons.
It is a quick and easy to do, here is a step-by-step guide with pictures.
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How To Enable Unknown Sources in KODI 17 Krypton
Step 1) Click the Settings Icon from the top left
Step 2) Click the System Settings box
Step 3) Expert Mode is needed and can be changed by clicking the box.
Step 4) Click the Add-ons button
Step 5) Click the Unknown Sources slider bar
Step 6) A warning box will come up click YES
Add-ons will now be able to be installed. To go back to the main menu right click a empty space on the screen, or click the top left Settings / System.