How to write plug-in for Windows Messenger 5.1

[Jim]

Hi,

I want to know how to write a plug-in for windows messenger 5.1.

I found that we can write a com component implementing IMessengerAddIn.
however i'm unable to find out the API providing this interface.

Following link on msdn gives a brief about the API's supported for Windows
messenger 4.6 or 4.7/
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/WinMessenger/winmessenger/messenger_entry.asp

But I'm unable to find any API for download and also if I can use the same
for Windows messenger 5.1.

Any help would be appreciated

Thanks