The latest source distribution can be downloaded from here, but you'll probably want to get a pre-built package for your platform if possible.
If you're running Debian (or Ubuntu etc) then you can install packages directly from the repository here. That will ensure that the Ruby and GTK dependencies are resolved automatically.
The manual installation instructions describe all the files in the distribution and where you need to put them on your system to make them work. This information is really targetted at people who wish to build packages for other OS distributions.