var stage = new Kinetic.Stage({container:'container',width:500,height:500});
var Layer = new Kinetic.Layer();
var myRectangle = new Kinetic.Rect({x:100,y:30,width:100,height:50,fill:"red",stroke:'black',strokeWidth:3});
Layer.add(myRectangle);
stage.add(Layer);
this code must show a rectangle in the div with id "container" but nothing shown in the page.
Works okay for me: http://jsbin.com/vutura/4/edit
I wonder if your code is loading before the element is on the page?
Maybe paste the code at the bottom of your page? Also make sure that you are including the Kinetic Library.