CODE 17.3 Krypton has changed from the old interface to a new look. It also has several options including changed now need to enable Unknown sources add-ons to install.
It is a quick and easy to do, Here is a step-by-step guide, to allow as unknown source on your kodi.
To enable Unknown sources in KODI 17.3 Krypton
step 1) View on your KODI Home, click on the Settings Icon from top left
step 2) click on the System Settings field
step 3)Expert Mode is required , and can be changed by clicking on the box.
step 4) click on the Add-ons – Taste
step 5) Click on the Unknown Sources slipway
step 6) an alert box will come , click YES
Add-ons can be installed now. To return to the main menu, right-click an empty area on the screen, or click on the upper left settings / System.
That's it, now enjoy your favorite KODI Addons: D