jQuery Grid Plugin - jqGrid

 I'd like to get the html '<select>...</select>' from the GetGradeDescList method of the StartingPoint controller (blue highlight below) to populate the dropdown listbox when editing the row, but cannot figure out the correct invocation to pass the parameters.  What am I missing?

Code:

    function loadGrid() {

        var blend = $("#Blends").val();

        var urlstring = '<%= Url.Action("GetStartingPoint", "StartingPoint")%>' + '?blend=' + blend;

        var gradeTypeCd;

        var gradeCd;

        var units;

        $('#fu').jqGrid({

            url: urlstring,

            datatype: 'json',

            mtype: 'GET',

            colNames: ['Id', 'Grade Type Cd', 'Grade Cd', 'Units', 'GradeDesc'],

            colModel: [

                { name: 'Id', key: true, index: 'Id', width: 20, align: 'left', editable: false, hidden: true },

                { name: 'GradeTypeCd', key: true, index: 'gradeTypeCd', width: 150, align: 'left', editable: false },

                { name: 'GradeCd', index: 'gradeCd', width: 100, align: 'left', editable: false },

                { name: 'Units', index: 'units', width: 50, align: 'left', editable: false },

                { name: 'GradeDesc', index: 'gradeDesc', width: 250, align: 'left', editable: true, edittype: 'select',

                    editoptions: { dataUrl: '<%=Url.Action("GetGradeDescList", "StartingPoint")%>' + '?blend=' + blend + '&gradeTypeCd=' + gradeTypeCd + '&gradeCd=' + gradeCd + '&unit=' + units },

                }

            ],

            pager: $("#pager"),

            viewrecords: true,

            height: 400,

            width: 600,

            toppager: true,

            rowNum: 50,

            rowList: [10, 20, 50],

            imgpath: '~/Scripts/Themes/sand/images',

            caption: 'View/Edit Starting Points',

            gridview: true,

            jsonReader: { repeatitems: true, id: "Id", cell: "cell", root: "rows", page: "page", total: "total", records: "records" },

            onSelectRow: function (id) {

                var grid = jQuery('#fu');

                var row_id = grid.jqGrid('getGridParam', 'selrow');

                gradeTypeCd = grid.jqGrid('getCell', row_id, 'GradeTypeCd');

                gradeCd = grid.jqGrid('getCell', row_id, 'GradeCd');

                units = grid.jqGrid('getCell', row_id, 'Units');

                //alert("blend = " + blend + ", gradeTypeCd = " + gradeTypeCd + ", gradeCd = " + gradeCd + ", unit= " + units);

            }

        }).navGrid("#pager", { edit: true, add: false, del: false, refresh: true, search: false },

        {

            url: '<%= Url.Action("EditStartingPoint", "StartingPoint")%>',

            onclickSubmit: function (params, postdata) {

                postdata.BlendCd = $("#Blends").val()

            },

            closeAfterEdit: true,

            reloadAfterSubmit: true,

            onSubmit: function (response) {

                alert(response.responseText);

            }

        }

      );

    }

Controller Code:

        public string GetGradeDescList(string blend, string gradeTypeCd, string gradeCd, int unit)

        {

            string selectList = "<select>";

 ... string concatentation removed for brevity ... 

            selectList = selectList + "</select>";

            return selectList;

        }