I think you're just having a naming problem.
Make sure you've named the listbox.
Go in the Properties menu (if you are using MX) it should say something like Component and a text input under that. In the text input it should say <Instance Name>, delete that then enter Listbox1, then try this.
ListBox1._visible = true; // this makes the list box disappear
ListBox1._width = 10; // this sets the width to be 10
ListBox1.addItem("this is a label", "this is data"); // doesn't do anything