set mmail-path

SET MMAIL-PATH filename  -  locate the gnu emacs library for mm
Usually:       /usr/local/lib/mm/mmail.el

The MMAIL-PATH variable defines the file of elisp functions used by gnu 
  emacs in MM.  This variable is relevant only if you SET EDITOR to emacs
  and SET GNUEMACS-MMAIL YES.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set editor   set gnuemacs-mmail