how to instantie movie clip

Link the library item to ActionScript by ( selecting the symbol in the library > right click > Properties > in the symbol properties window select Export for ActionScript check-box > in the Class field type the class name for example myMc or keep it as it's and finally click OK.

To add the item to the stage:

var _mc:MovieClip = new myMc();

_mc.x = 100;

_mc.y = 50;

addChild(_mc);