i dont understand this point: help me please, how i do it?
At this point the client app is ready to receive a remote notification message. From the command line, run the MessageSender.exe program to send a "Hello, Xamarin" notification message to the client app. If you have not yet built the MessageSender project, do so now.
To run MessageSender.exe under Visual Studio, open a command prompt, change to the MessageSender/bin/Debug directory, and run the command directly:
To run MessageSender.exe under Xamarin Studio on the Mac, open a Terminal session, change to MessageSender/bin/Debug the directory, and use mono to run MessageSender.exe