As pointed out above my sogorman, When you added the imageviewer control to a panel or a container it doesn't show.

i too faced the same problem.

i had to make the the imageviewer control modal property as true OR set top:1 for the image to show.

But once the image show, the drag, scrollers work as expected but the pinch to zoom doesn't work.
Code:
Ext.Loader.setConfig({
    enabled: true,
    paths: {
        'Ext.ux': '../../ux'
    }
});
 
Ext.application({
 
    name  : 'Simple ImageViewer Demo',
    views : [
        'Ext.ux.ImageViewer'
    ],
    launch: function() {
 
                        var container = Ext.create('Ext.Panel',{
                        height:'100%',
                        width:'100%',
                        // modal:true,
                        items: [
        {
                                    xtype: 'imageviewer',
            style: {
                backgroundColor: '#333'
            },
                                    modal:true,
                                    height:'99%',
                                    width:'100%',
            imageSrc: 'http://farm8.staticflickr.com/7242/7...ea1_o.jpg'
        }
    ]
                        });
                       
                        Ext.Viewport.add(container);
           
       
                       
                       
                       
                       
    }
});