In order to add a MovieClip from the library to the Stage with JSFL you would use:
fl.getDocumentDOM().library.addItemToDocument(position [, namePath]);
The position parameter is the x, y coordinate where you would like to place the symbol on the stage.
The second parameter is a string value that specifies the name of the item in the library. If the item is inside of a folder you can specify the path using slash notation. If you don't include this parameter Flash will default to the currently selected item in the library.
Example: If there is a symbol called "myMovieClip" inside of a folder called "myFolder", and you wanted to place that symbol on the stage at x:250, y:180 the code would be:
fl.getDocumentDOM().library.addItemToDocument({x:250, y:180}, "myFolder/myMovieClip");
The documentation for doing this is on this page in case you want more information:
Hope that helps out.