Be sure to insert full version of jQuery UI. Also you should init the dialog first:
$(function () {
$( "#dialog1" ).dialog({
autoOpen: false
});
$("#opener").click(function() {
$("#dialog1").dialog('open');
});
});
<script src="https://code.jquery.com/jquery-1.11.1.min.js"></script>
<script src="https://code.jquery.com/ui/1.11.1/jquery-ui.min.js"></script>
<link rel="stylesheet" href="https://code.jquery.com/ui/1.11.1/themes/smoothness/jquery-ui.css" />
<button id="opener">open the dialog</button>
<div id="dialog1" title="Dialog Title" hidden="hidden">I'm a dialog</div>