ottimo, ho risolto!
codice:
numeroRandom = int(4*(Math.random()));

var randomString:Array = new Array();

randomString[0] = "btn_uno";
randomString[1] = "btn_due";
randomString[2] = "btn_tre";
randomString[3] = "btn_quattro";
randomString[4] = "btn_cinque";

mc_btns[randomString[numeroRandom]].onPress();

trace("btn pressed: " + randomString[numeroRandom]);