The image you are seeing indicates that the maps are not supported in your region. This is usually due to geopolitical issues. You can get around this by setting the HomeRegion and culture of the map like this:
<bm:Map HomeRegion="US" Culture="en-US"/>
I'm not sure if this is still the case with Windows 8.1. Also, I highly recommend targeting Windows 8.1 rather than Windows 8. The Windows 8.1 version of Bing Maps has a bunch of bug fixes in it.