Vanilla 2

Hi, I have a problem with the plugin.
It's uploaded to the server and I put those lines in the header.php file (I use wordpress):






$(document).ready(function(){
$("a[rel^='prettyPhoto']").prettyPhoto();
});



I get this error:


$(document).ready(function(){
Uncaught TypeError: Property '$' of object [ Object DOMWindow ] is not a function
$("a[rel^='prettyPhoto']").prettyPhoto();
});



How can I solve it?
Thanks in advance,
Jason