I think I know what you are talking about. (maybe)  Try creating a foreground terrain.  Then a background terrain that you have moved very far back from your view point.  You will probably have to stretch the back ground terrain along the x and y axis to fill the screen OR create or duplicate it and so you can spread the background terrains to cover your screen right and left. 

THEN I would group the backgrounds and tilt them ever so slightly to cover that horizon line.

You can never create a terrain and move it all the way to the horizon line.  In real life the horizon is visualized as the point where the curved earth falls from view.  In Bryce world the world is flat, not round, and it goes on into infinity. You will never be able to move terrain to the ends of the earth in Bryce.  I will try to post an example.