What is the easiest way to install emacs-ess on OSX?
Use the latest .tgz package:

Here is the installation version from the documentation (the -12.09-2 part of the file name is current-release specific and will change):

wget http://ess.r-project.org/downloads/ess/ess-12.09-2.tgz
tar -xvf ess-12.09-2.tgz
mv ess-12.09-2 /Applications/Emacs.app/Contents/Resources/site-lisp/
echo "(require 'ess-site)" &gt;&gt; ~/.emacs