Layer set visible

Question

I add this layer like this:

var POIData = new esri.layers.ArcGISDynamicMapServiceLayer("https://mysite.com/ArcGIS/rest/services/POI_Data/MapServer");
                    map.addLayer(POIData);
                    POIData.hide();

I want to show it from a button:

This is the button:

<div id="POIData" data-dojo-type="dojox.mobile.ToolBarButton" style="float: left;"onClick=" showPOI();" >POI</div>

And this is the function behind it:

function showPOI(){
POIData.show();

}

What am i doing wrong?

Any ideas. I just now cant see it, and cant set it to visible.

Thanks in advance