// METHOD 1
// Add Content
var someMc:String = "this_mc";
var newClass = getDefinitionByName(someMc);
var mcAdded= new newClass ();
addChild(mcAdded);


//  Use Content
var someString:String = "mcAdded";
var selectedInstance = getChildByName( someString);

selectedInstance.x = 210;

// METHOD 2
// In the MovieClip properties, set it to export for ActionScript and give it a class name.

//In your code.
var MC = new yourMCclassName();

// Add Content
var someMc:String = "this_mc";
var newClass = getDefinitionByName(someMc);
var mcAdded= new newClass ();
addChild(mcAdded);


//  Use Content
var someString:String = "mcAdded";
var selectedInstance = getChildByName( someString);

selectedInstance.x = 210;

Add content to stage from Library through code and use the content.

Load content from library onto stage

ActionScript 3