Error with custom button that works with apex method

I have a custom button in a salesforce page, and I want that when I click it, an apex method is executed. I follow the tutorial here but I get an error when I click it:

{faultcode:'soapenv:Client', faultstring:'No operation available for request {http://soap.sforce.com/schemas/package/DevisToFacture}createFacture, please check the WSDL for the service.', }

Here is the javascript I put when I created the custom button :

{!requireScript("/soap/ajax/29.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/29.0/apex.js")}
var result = sforce.apex.execute("QuoteToInvoice","createInvoice",{idQuote:"{!Quote.Id}"});
alert(result);
window.location.reload();

My apex class is named QuoteToInvoice (it is a public with sharing class), the method is named createInvoice and it has a parameter idQuote whose type is Id.

Do you have any idea of what I'm doing wrong ?