Como instalar o modo ac-R no emacs?

Question

Eu adicionei recentemente o arquivo ac-R.el to /usr/local/share/emacs/site-lisp , junto com (require 'ac-R) to ~/.emacs

Agora, quando abro o emacs com o --debug-init, recebo o erro

    Debugger entered--Lisp error: (void-variable ac-modes)
  add-to-list(ac-modes ess-mode)
  eval-buffer(#<buffer  *load*<2>> nil "/usr/local/share/emacs/site-lisp/ac-R.el" nil t)  ; Reading at buffer position 7191
  load-with-code-conversion("/usr/local/share/emacs/site-lisp/ac-R.el" "/usr/local/share/emacs/site-lisp/ac-R.el" nil t)
  require(ac-R)
  eval-buffer(#<buffer  *load*> nil "/home/dlebauer/.emacs" nil t)  ; Reading at buffer position 3548
  load-with-code-conversion("/home/dlebauer/.emacs" "/home/dlebauer/.emacs" t t)
  load("~/.emacs" t t)
  #[nil "54

e ao clicar em load-with-code-conversion , diz

Can't find library /usr/share/emacs/23.1.50/lisp/international/mule.el

apesar de ter instalado o mule via synaptic (estou usando o Ubuntu 10.04)

Como posso obter a biblioteca de mulas no lugar certo?