Messenger Plus! Live is not a "traditional" addin like the ones you can see in the Addins tab of the Options window. It uses a different method of communicating with Messenger, by actually hooking onto it.
Unfortunately, now that Addins have been apparently removed (I didn't think they served much purpose anyway, seeing as I had to make a registry tweak to make the tab even appear) I doubt you'll be able to make an addin that way. There are probably plenty of articles out there that tell you how to hook onto Messenger using .NET, though.
Hope this helps
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