Xamarin.Forms.Map: empty box instead of the map

Hi,

I'm a newbie in the Xamarin world. I would like to use the Map component of Xamarin.Forms but it doesn't work as expected: I see an empty box instead of the map.

Here the code in my PCL project:
public class App {
public static Page GetMainPage() {
return new ContentPage
{
Content = new Map
{
VerticalOptions = LayoutOptions.FillAndExpand,
HorizontalOptions = LayoutOptions.FillAndExpand,
BackgroundColor = Color.Red,
MapType = MapType.Street
}};}}

The red background is coming in my iOS & winPhone projects, but not the map :-( What's wrong with my simple code?

Is there any tutorial for Xamarin.Forms.Map? I'm searching a tutorial for the cross platform component, not the iOS or Android specific component...