Widget Windows - consume Web Service REST with JSON response

Widget Windows - consume Web Service REST with JSON response

I implement a gadget windows and I can call a REST Web Service. I have
search on many topic in google and in stackoverflow, but I haven't fund
the solution of my problem.
I want to use an ajax call, but it doesn't work, the "$" isn't defined.
My code :
function callWebService(){
$.ajax({
type:"GET",
url:"url",
data:'arguments',
dataType:"json",
success : function(msg){
loadJSON(msg);
},
error: function(msg){
alert("Error : " + msg);
}
});
}
Thank you in advance