Trouble with JSON

Trouble with JSON

I searched a bit on the forum but the problem remains, maybe it's simple, but I haven't worked with JSON + jQuery before:

For example:
when I try to do this
$.ajax({
url: 'http://isohunt.com/js/json.php?',
   dataType: 'json',
success: function(data) {
alert('Worked');
alert(data);
}
});
data is null, it isn't supposed to be right? how do I actually retrieve the json data from the url?