Thanks to Trappski for his great tutorial. Now I'm just adding a little extra, without screenshots or anything to tell you how to get the latest plugins (and skins) from amsn's SVN.
First, for those who don't know what SVN is, I suggest you read this : http://en.wikipedia.org/wiki/Subversion
simply said, it's the way, we, amsn's developpers, use to share the development version's code. Which means SVN has the latest features and bug fixes that amsn could have.. but it can also contain new bugs (pertaining to new features). You could find in the latest SVN version, features such as voice clips support and offline messaging, amongst others.
Now, if you want to get your plugins from SVN too (which has a huge database of plugins), simply go to the directory :
C:\Documents and Settings\YOUR_WINDOWS_USERNAME\amsn\plugins
where YOUR_WINDOWS_USERNAME is your windows username (not the msn username, but the windows username...)
Then right click in that directory, select SVN checkout, and enter the URL :https://svn.sourceforge.net/svnroot/amsn/trunk/amsn-extras
Then let it check it all out for you in that current directory.