I was having the same issue as most people here... I installed it and it says some text under the title saying that it will search for similar threads... but when I type something into the title box nothing happens.
The reason seems to be that you need the SPECIFIC version of jquery that the author uses because he probably hardcoded the call to it.
SOLUTION:
DOWNLOAD THIS VERSION OF THE JS FILE AND PUT IT IN YOUR CLIENTSCRIPTS FOLDER
http://code.google.com/p/jqueryjs/do....1.3.1.pack.js
Should work now
Also, you need to CLICK INSIDE THE MESSAGE BODY AREA to trigger the ajax check