The RecieveNotify function provided by the Messenger Plus! API doesn't capture messages send/recieved. This function is used to allow two of the same plugins (one on each computer) to react a specific way to a command send that
requires the contact to respond. Take the /ping command for example. You can do this using RecieveNotify. Messenger Plus! will see the character to specificy its for RecieveNotify then it will check against 5 specific characters to verify the plugin is supposed to respond.
If you wait for Messenger Plus! Live it has OnEvent_ChatWndReceiveMessage which is trigered when a message is sent/recieved.
Alternatly you can use ActiveAccessibility :
Matty's reply to Active Accessibility for incoming messages.