It's completely normal that you're having issues. The official Arc documentation is...lacking.
The "latest" arc is Arc 3.1, but that came out in 2009. You can download it here: http://arclanguage.org/item?id=10254, and start it up with `racket -f as.scm`. (Note that you no longer need the specific version of MZScheme you used to; you can use racket.) No, this information has not been updated on the official Arc install page. Yes, this is frustrating.