First, make sure you're trying to use the right app. If you have a Slingbox, and are trying to access it, you want the Slingplayer for Fire TV app, not the SlingTV app.
Here is a little information on our app just for reference:
However - if you are trying to access the SlingTV service, and their app is not working for you, then you are in the wrong support forum. You will want to look for Sling TV's support line.
Hope this helps!