This help page is for version 3.4. The latest available help is for version 5.1.
The Network Message Action is equivalent to doing a "net send" from the command line. It allows you to direct a message box pop-up to any particular user or computer on the network.
The client machine must be running Microsoft's Messenger service to receive and display these messages. Because of spam and security concerns, the Messenger service is not started by default on most systems.