To use MultiHack, go to
http://www.multihack.com , download and install it, and double-click on the file you downloaded. There will be a few things you'll have to set up, such as the base scripts directory, but this isn't too hard. After that, just find the hack's name in the list that appears and hit "Install".
(I know these instructions are a little sketchy, I'll write better ones soon
)
