I just tried it on Mac OS X 10.6.4 with a fresh installation of Firefox 4.0.1 and Greasemonkey 0.9.3, and the script worked fine. I then installed NoScript, and my script still worked under default settings. There was no need to install the script manually. The script does not require any LW privileges, and in fact works even if you're not logged in (except of course the vote up/down buttons won't work).

I'm not sure what else might be causing the problem, but http://wiki.greasespot.net/Troubleshooting_%28Users%29 has more troubleshooting steps you can try.

(BTW, the reason you see "The page you requested does not exist" is that I picked an arbitrary non-existent LW page as the "trigger" for my script so the fact you see that message means the script either didn't get run, or it failed to do anything.)