MooTools Request

[Cristisor]

Hello. I'm trying to make a request to the server using MooTools and I
don't know why, but it doesn't work. This is my request:
        var myRequest = new Request({url : _url,
                                data : _data,
                                onSuccess: function() {
                                        alert('Merge');
                                },
                                onFailure: function() {
                                        alert('Nu merge');
                                }
        });
        myRequest.send();
I'm using the 'post' method as the default. Every time it goes to the
onFailure branch. I used prototype until now, so I'm new to MooTools.
Thank you!