IMPORTANT:
As of version 1.0.1, this widget has been removed from the Ajax Event Calendar plugin. Instead, use the [eventlist] shortcode explained here:
http://wordpress.org/extend/plugins/ajax-event-calendar/installation
The shortcode offers more customization and placement options than the old widget, explained here:
http://code.google.com/p/wp-aec/wiki/ShortcodeOptions