If you are reading this, you probably just downloaded the jQuery.mmenu-plugin and are eager to learn more about it.
If so, these documentation pages could not be more helpfull. They include examples, a tutorial, the full documentation for the configuration options and custom events, some color themes and even a demo on how to implement it with jQuery Mobile.
This page has the mmenu-widescreen.css file included. Therefor, if your screen is wider than 900px, the menu will always be opened.
If not, click the menu-button in the header to open the menu.