Were can I download the Messenger Add-In SDK
There is no download link at the MSDN.
I did'nt find too, but not need. If you want develop any add-in for the Messenger only two thing need:
1st: the reg key: AddInFeatureEnabled dword type value set to 1 or you must create them if it's not exist.
2nd: Visual Studio 2005
see this link: http://blogs.msdn.com/katieblanch/archive/2006/05/11/595765.aspx and follow the description, and the example code.
I already did it and it works.
I don't understand why MS don't update the relevant MSDN documentation.
It is also on msdn. go to http://msdn.microsoft.com/library/default.asp url=/library/en-us/dnlive/html/messengeraddin_sdk.asp
Also check out the latest channel 9 video with the dev who worked on addins in the video talking about them