// In my case I want to load them onload, this is how you do it! // 
Event.observe(window, 'load', loadAccordions, false);

// Set up all accordions //
function loadAccordions() {
	var topAccordion = new accordion('horizontal_container', {
		classNames : {
			toggle : 'horizontal_accordion_toggle',
			toggleActive : 'horizontal_accordion_toggle_active',
			content : 'horizontal_accordion_content'
		},
		defaultSize : {
			width : 807
		},
		direction : 'horizontal'
	});
	// Ouvre Onglet 'accueil'
	topAccordion.activate($$('#horizontal_container .horizontal_accordion_toggle')[6]);
}