I don't believe that you can add a symbol straight out of the library.
You have to give the symbol a linkage (class name) then create a new instance of the class, then add that instance as child.

IE.
var scrollbox:MovieClip = new MovieClip();
this.scrollbox.x = 50;
this.scrollbox.y =50;
this.scrollbox.width = 340;
this.scrollbox.height =520;

var sb1 = new sb(); // sb being the linkage name in the library
this.scrollbox.addChild(sb1); // you then reference it by the "new" name
this.addChild(scrollbox)