1. Put json.el and mojo.el somewhere in your load-path.
    (Use M-x show-variable RET load-path to see what your load path is.)

      => http://github.com/samsonjs/mojo.el       -- mojo.el and json.el
      => http://edward.oconnor.cx/2006/03/json.el -- official json.el
 
 2. Add this to your Emacs init file: (require 'mojo)
 
 3. Enable mojo-mode for modes that you use for webOS, e.g.:
 
    (mojo-setup-mode-hooks 'css-mode-hook 'js2-mode-hook
                           'espresso-mode-hook 'html-mode-hook)
 
  * Note that this does not simply enable mojo-mode for these types
    wholesale, but instead only enables mojo-mode when it finds that
    the file is also under a Mojo project root (using mojo-project-p).

 4. Make sure you customize the variables:
    mojo-project-directory, mojo-sdk-directory and mojo-build-directory
    (Use M-x customize-group RET mojo RET)

 (optional but recommended)

 5. js2-mode for JavaScript and espresso-mode for JSON.

      => http://code.google.com/p/js2-mode/
      => http://www.nongnu.org/espresso/
 

That's it! You've got the most powerful Mojo development environment.

Check COMMANDS for a list of all interactive commands and key bindings.