I have recently added the file ac-R.el to /usr/local/share/emacs/site-lisp, along with (require 'ac-R) to ~/.emacs

Now, when I open emacs with --debug-init, I get the error

    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 "\205\264

and when clicking on load-with-code-conversion, it says

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

even though I have installed mule via synaptic (I am using Ubuntu 10.04)

How can I get the mule library in the right place?