There isn't actually much use for that userscript (at least not exactly as coded) since the two browsers where it would work are Firefox and Opera and both those browsers have a better way of overriding any attempt to block access to the context menu.
In Opera go into the Tools menu and then Preferences and on the Advanced tab select Content and then press the JavaScriipt Optiions button and check that the option to Allow scripts to detect context menu events is unchecked.
For Internet Explorer, Safari, and Chrome you could use a bookmarklet version of the code to add an entry to "Re-enable Context Menu" into the browser Favorites/Bookmarks menu so you can override "no right click" scripts on a page by page basis.
With Internet Explorer 6+ and the IE7Pro plugin you could add a slightly modified version of that userscript into the browser (as the userscript option in IE7Pro uses a slightly different format to what Firefox/Greasemonkey and Opera use). There are other plugins that allow userscripts to be added to IE but that's the only one of the three or four that I tried that actually works properly.