How do I make 'Reboot Notification' work?

Reboot Notification (also known as an alert) requires that the Messenger service, a standard Windows OS service, be started on the Master-Agent and any target you are trying to notify.

The "NET SEND <target hostname> <message>" command also uses the messenger service. Once you believe the messenger service has been started on the target, it is easy to test notification by using NET SEND as shown in the screen shot below.

Notice that when messenger is turned off, you get:

If you have many targets where the messenger service is off, and you wish to start them, you can use the UpdateEXPERT custom fix feature to deploy a command to start the service. See the Custom Fix overview article ... there are links to starting simple services like Telnet or Messenger.