It's in a branch for the old Arc 2, so it probably won't work with Arc 3.1. It should provide a starting point, though; let us know if you get it working, and feel free to push it to http://github.com/nex3/arc/tree/master.
b) Run the shell script, install it to the directory containing the arc2.master branch.
plt/bin/mzscheme --no-init-file --load as.scm
It seems to come up without errors and ffi.arc seems to be loading up.
(When you report errors it's usually much faster to tell people how to reproduce your error in this step by step manner. Then they can usually immediately look at the instructions and tell you what you're doing wrong without a lengthy back-and-forth.)
Can you try the steps I outlined and confirm that it works for you? Perhaps something's messed up about the version you're on. Once you have a working version we can go in and poke at the differences to see where the problem might be.
That's a good approach. Keep at it, I'm sure you'll figure it out.
One way to get help with it: describe simple step-by-step changes to the repo that cause errors. Download arc 3.1, add this file from here, type this in at the repl, etc. Don't forget the actual text of the error you see as well.