This doesn't work for me either, either with the script as-is or with the .tld-s changed to .com-s.
In the error console I get messages "Permission denied for <https://mail.google.com> to get property Window.addEventListener from <https://www.google.com>."
Changed .tld as recommended by others, doesn't seem to work now. Maybe because of some google labs feature I have enabled, but I doubt it.
I only had to tweak the Included Pages in the configuration (Tools -> Greasemonkey -> Manage User Scripts...), and change the ".TLD" to ".COM" in the GMail urls.
(I assume they did this for non-US users...)