This is a little more complicated as the software is not in the Amazon Store, but if you follow these steps you will install it easily.
Firstly you need to hover over the settings on the main menu of your Firestick
and select My Fire TV
and then select developer options,
and then apps from unknown sources (which will be off) and click to turn it on
You will receive a warning
so just select Turn On.
Return to the Main Menu and hover over the search options and type in Downloader
This will bring up the downloader app on your screen
Select the Downloader App and you may have to agree to give the app permission - just select Allow
Next type in the URL (address) exactly as below:
wait for the file to download
and when it has finished click install
and you have installed the IPTV software which will appear on your menu
Now you have just to tell the software your username and password, so open the app.
When you first open it you will need to add a user, with the details you have received.
The first line is any name you wish to give the user.
The second line is the username you have been given.
The third line is the password you have been give,
The fourth line is the server address you have been given.
When you have filled these in simply press "ADD USER" and if you did not make a mistake the software will download lists from the server.
When this is completed you will be shown the main menu of the software and you are ready to go!