Attack.arena_text(string) is the method used by the game to display the current round (On the top of the screen, the label "Round X" that fade in then fade out). I don't think it is the right one.
I found an interessant link thanks to yours :
http://kingsbounty.ru/docs/scripting
Yes its in russian, but with google translator, it is useful for modders. But it is not complete (i didn't find Attack.arena_text(string) method for example).
According to you, if I wanted to display an additional label with Attack.arena_text(string), where and in which file should I place this method ? At the end of the file ?