[7thsky™]

this.createEmptyMovieClip("my_mc", 0);
my_mc.createTextField("my_txt", my_mc.getNextHighestDepth(), 0,10, 300, 100);
my_txt.border = true;
my_txt.multiline = true;
my_txt.wordWrap = true;
var my_fmt:TextFormat = new TextFormat();
my_fmt.color = 0x00FF00;
my_fmt.underline = true;
my_txt.text = "This is my first test field object text.";
my_txt.setTextFormat(my_fmt);

а откуда ты взял my_txt?