Unfortunately I have to tell you that you won't find the plugin here anymore. The needed functionality has long been integrated in the Wordpress core, so there was little need for further development, which is why this page and the plugin have been abandoned for some time.
I would strongly advise you to upgrade to the current Wordpress Version and use the provided comment threading features.
In case you still need the plugin for any reason you can get it from the Wordpress Plugin Directory:
Thanks for all your downloads and take care