Using Direct Download URL
Be sure you have “Apps from Unknown Sources” turned on.
For the new Fire TV interface, this is underSettings > Device > Developer Options
For the old Fire TV interface, this is underSettings > System > Developer Options
Install Downloader from Amazon’s Appstore onto your device. You can find it by simply searching for “Downloader” on your device.
Launch Downloader and enter the URL of the APK file you want to install.
For Radiosity: MAKE SURE YOU USE THE URL BELOW, NOT THE ONE IN THE PICTURE
Select the “Download” button and wait for the APK file to download.
Once the file finishes downloading, it will automatically ask to be installed. Press down a few times on your remote to select the “Install” button at the bottom right of the screen.
When the APK file finishes installing, you can select to delete the APK you downloaded now that the app is installed.