Tutorial for writing a widget for multisite

Hmmm, I wonder why I can’t get any examples to show up in my available widgets list. One tutorial said to use:

class My_Widget extends WP_Widget {
    function My_Widget() {
        // widget actual processes
    }

    function form($instance) {
        // outputs the options form on admin
    }

    function update($new_instance, $old_instance) {
        // processes widget options to be saved
    }

    function widget($args, $instance) {
        // outputs the content of the widget
    }

}
register_widget('My_Widget');

and save the php file in my wp-content/plugins folder which I did but it doesn’t show up anywhere (yes I know the plugin doesn’t do anything but I’m setting my sights low and trying to just get it to show up first).