The code below implements the navigating feature using the 'LEFT' and 'RIGHT' keys.
<script type="text/javascript">
(function($){
	function nav(delta){
		var win = $('body').children('div.messager-window');
		if (!win.length){return}
		var buttons = win.find('.messager-button .l-btn');
		for(var i=0; i<buttons.length; i++){
			if ($(buttons[i]).is(':focus')){
				var index = i + delta;
				if (index > buttons.length-1){
					index = 0;
				} else if (index < 0){
					index = buttons.length-1;
				}
				$(buttons[index]).focus();
				return false;
			}
		}
	}
	$(document).bind('keydown', function(e){
		if (e.keyCode == 39){	// right
			return nav(1);
		} else if (e.keyCode == 37){	// left
			return nav(-1);
		}
	})
})(jQuery);
</script>