On the home screen, go to the magnifier (Find).
Type MaxPlayer, or hold the Alexa button and say "MaxPlayer".
Pick MaxPlayer and select Get (or Download).
Open MaxPlayer and sign in with the username and password you received.
No Downloader or code needed: MaxPlayer is in the Amazon store. The Downloader codes going around online install a different app.
Works on Fire TV Stick with Fire OS (2nd gen or newer) and Fire TV Cube.