Comment 2 for bug 499885

This is not *technically* a bug with the extension itself. This is caused because the browser does not recognize the contents of the "div" as a block item. Because it is not recognized as a block item it is given no default height and does not display. Here is the fastest solution for you to use.

Create a new CSS style:
#map_canvas {
    width: 500px;
    height: 300px;
}

That will force the browser to give the div a width/height and the contents of the div will show up. I have run into this same thing before with map generation before, I don't know why I didn't think to document the behavior.

I will update the documentation on the Yii website to make a note of this behavior. I also have created a feature request for the ability to specify the width & height of the map canvas div when you render the map.