I've made some minor changes to your code. In fact I've added a few properties to your layer:

    var skoLayer = new OpenLayers.Layer.XYZ("skobbler",
    				["${z}/${x}/${y}.png?api_key=<Your Key Here>"], 
    					{numZoomLevels: 18, 
    					sphericalMercator: true, 
    					isBaseLayer: true, 
    					wrapDateLine: true, 
    					maxExtent: new OpenLayers.Bounds(-180, -180, 180, 120),
    					attribution: attrib
    				});



And **[here][1]**'s the whole code for you, to easily test it. Just unzip the archive (*tested for  viruses*), extract the *scrobbler.html* file and double click it.

Just zoom out and move the map, as you like.


  [1]: https://dl.dropboxusercontent.com/u/13060205/Stackexchange/Skobbler/skobbler.zip