To edit Mathemagix code with Emacs, a mmx mode can be used. It is available in mmxlight/emacs/mmx-mode.el and can be installed as follows:

cp mmx/mmxlight/emacs/mmx-mode.el $HOME/.emacs.d

The following can be added in the file .emacs:

(setq load-path 
  (append load-path (list (expand-file-name "~/.emacs.d"))))
(setq auto-mode-alist 
     (append auto-mode-alist '(("\\.mmx" . mmx-mode))))
(autoload 'mmx-mode 
     "mmx-mode.el" "Major mode for editing Mathemagix files" t)

Once this is done, the mmx-mode will be loaded automatically for files with suffix .mmx.