Step 1= From the home screen of your device, hover over the Find option.
Step 2= Click Search.
Step 3= Search for and select Downloader.
Step 4= Choose the Downloader app.
Step 5= Click Download.
Step 6= Wait for the app to install.
Step 7= Once the app finishes installing click Open.
Step 8= Return to the home screen and open Settings.
Step 9= Click My Fire TV.
Step 10= Choose Developer options.
Step 11= Click Install unknown apps.
Step 12= Find the Downloader app and click it.
Step 13= This will turn Unknown Sources to On for the Downloader app. This will enable side-loading on your device.