The debugger says that the deviceToken is null but it is not. (At least that is what I observe) You can use the code below to build a string that represents your device token.
public override void RegisteredForRemoteNotifications (UIApplication application, NSData deviceToken)
{
byte [] token = deviceToken.ToArray ();
string tokenString = "";
for (int i=0; i<deviceToken.Length; i++)
tokenString += token[i].ToString ("X2");
Console.WriteLine (tokenString);
}
Here is a good tutorial that explains push notifications. It is for XCode but it is easy to convert.