I just watched the kings moot again and still no horn. If the show were going to use it (they might) they would have featured it a bit more prominently. My theory is that realizing his Brice has beat him to Dany he will ally with Cersi.
You don’t have to see the horn to witness the effects.
Boring television/writing: “Here’s this awesome horn which will mind control everyone and get them to do what I say when you, Person I Have No Use For or Maybe Need Out of my Way, blow it for me and proceed to fall over dead afterwards” followed by a long, knowing gaze into the camera.
Watch the scene again, and pay attention to it. :)