Hi and sorry for not answering earlier.
The plugin itself it really simple, but the problem is that it's *that* simple because it was just including another package (binding) that was using Growl. Since 1.3, Grown dropped support for TCL (and a few more languages) and nobody rewrote the bindings that we were using.
I'm sorry to tell you that Growl plugin won't come back any time soon (unless some dev with some free time want's to send me patches).
Check out the TclGrowl package at http://www.codebykevin.com/opensource/oss.html
(scroll down the page a little bit) and see if that might work. It should be as simply as mapping the commands from the old Growl code to the new version: I don't think the commands are quite a drop-in replacement, but as the new version uses AppleScript and is works fine with the new version of Growl, it should work for aMSN. I will be glad to test a version that incorporates this code, although I don't have time to submit a patch myself.