The packages are on the Emacs MELPA package repository. Here is the updated screencast for the latest version. The live error messages that show up at the bottom are from
stanc (2) and
stanc3 likely coexist in the wild, the executable names are assumed to be
stanc3 by default. Please see the NEWS and Configuration. The new reserved words
multiplier have been included in the syntax highlighting. See the related discussion.