http://ycombinator.com/arc/arc3.1.tar, it includes news.arc, the code running HN/NewMogul/etc and a "how-to-run-news" text file.

You need MzScheme 4 to run Arc.

See also http://arclanguage.org/install (installation instructions, but a bit out-of-date, it's for Arc 3.0) and http://arclanguage.org/item?id=10254 (Arc 3.1 announcement).