StartGame.

StartGame.cs

var gamePackageName = Resources.GetString(Resource.String.game_package_name);
var gameIntent = PackageManager.GetLaunchIntentForPackage(gamePackageName);
if (gameIntent == null)
{
	try {
		StartActivity(new Intent(Intent.ActionView, Android.Net.Uri.Parse("market://details?id=" + gamePackageName)));
	} catch (ActivityNotFoundException) {
		StartActivity(new Intent(Intent.ActionView, Android.Net.Uri.Parse("http://play.google.com/store/apps/details?id=" + gamePackageName)));
	}						
}
else 
{
	StartActivity(gameIntent);
}