Firefox users need
Chrome users need
Once you have one of those, Scroll DOWN...
Scroll down a little bit more...
then press the 'Raw' button
at the right-side of the Dim.Twit.user.js script.
(normally clicking Raw would show you the raw text,
but GreaseMonkey and TamperMonkey recognize the text as a script,
so they ask if you want to install it.)