You need to ensure that you have a jQuery reference that is declared prior to you actually using any jQuery code as seen below : 

<!-- jQuery Reference (from CDN) -->
<script src="http://code.jquery.com/jquery-latest.js"></script>

<!-- Your jQuery Code -->
<script type="text/javascript">
    $(function () {
        //When an available seat is clicked on
        $('.seat.available').click(function () {
            //Toggle selection for this seat
            $(this).toggleClass('selected');
        });

        //When your "Purchase" button is clicked
        $("#Purchase").click(function () {
            //Grab the selected seats
            var selected = $(".seat.selected");

            if (selected.length == 0) {
                alert("No seats were selected.");
            }
            else {
                alert(selected.length + ' seats were selected. Proceed to Purchase screen.');

                //You may want to add some additional properties to store your seat ID within your
                //seat <div> elements so that you can read them here (look into HTML5 data-attributes)
            }
        });
    });
</script>