问无法使用ajaxSubmit : TypeError: Object [object Object]没有方法'ajaxSubmit‘

EN

<div id="dialog" style="display: none" title="Retrieve your password">
        <p>
            Please enter your username below and you will have a new password sent to the email address you registered with the account.
        </p>
        <form id="passform" action="comment.php" method="post">
            <input type="text" name="user" id="user">
        </form>
    </div>

$(document).ready(function()
        {
            function doit(formData, jqForm, options)
            {
                // formData is an array; here we use $.param to convert it to a string to display it
                // but the form plugin does this for you automatically when it submits the data
                var queryString = $.param(formData);

                // jqForm is a jQuery object encapsulating the form element.  To access the
                // DOM element for the form do this:
                // var formElement = jqForm[0];

                alert('About to submit: \n\n' + queryString);

                // here we could return false to prevent the form from being submitted;
                // returning anything other than false will allow the form submit to continue
                return true;
            }


            $("#getpass").click(function()
            {

                $("#dialog").dialog(
                {
                    autoOpen : true,
                    resizable : false,
                    height : 270,
                    modal : true,
                    hide : "fold",
                    show : "fold",
                    buttons :
                    {
                        "Retrieve Password" : function()
                        {
                            $("#passform").ajaxSubmit(
                            {
                                url : "php/lostpassword.php",
                                type : "post",
                                success : doit
                            });
                            return false;
                        },
                        Cancel : function()
                        {
                            $(this).dialog("close");
                        }

                    }
                });

            });

        });