post-mode has a new home and is now hosted on Sourceforge. You can download it from the Sourceforge project site.
Extract post.el with gunzip post.el.gz. Put post.el in your load-path and add the following snipplet to your .emacs file:
(require 'post)
A French howto was available but seems to have gone missing. Please update this page if you know where it is.
Otherwise, all documentation can be found on the Sourceforge project site.