在公司里写的用来共享的小笔记，就用英文了。
The mode info and download link can be found in www.emacswiki.com.
1. Install cc-mode
Make a directory named cc-mode under your /site-lisp directory of emacs.
Download cc-mode compress package and uncompress it.
Compile all the .el files according to its README document.
Move all the .elc files to the /site-lisp/cc-mode directory.
2. Install actionscript-mode
Download actionscript-mode.el and move it to the /site-lisp directory of emacs
3.Modifiy your ~/.emacs file and add the following content, note the site-lisp dir path may be different on your machine:
(add-to-list 'load-path "/usr/share/emacs23/site-lisp/cc-mode")
(when (require 'actionscript-mode nil t)
(setq auto-mode-alist
(cons '("\.as" . actionscript-mode) auto-mode-alist)))
Open an .as file and you can see the syntax highlight now.
For .mxml file, it uses nxml-mode by default (I'm using emacs 23.1.1), you can switch to actionscript mode by typing "M-x actionscript-mode RET" to read AS code, and switch back by typing "M-x nxml-mode RET" to read xml code. Currently there is no flex-mode.el available and I don't like the multi-major-mode idea.