var t = new mx.transitions.Tween (movieclip, "_x", mx.transitions.easing.Regular.easeOut, xinicial, xfinal, duracao_em_segundos, true);
ou
var t = new mx.transitions.Tween (movieclip, "_x", mx.transitions.easing.Regular.easeOut, xinicial, xfinal, duracao_em_frames, false); Obs.: Funciona no mx 2004