The page(datagrid.html) to be loaded cannot contain a body tag. Try the page code below:
<table id="tt"></table>
<script>
$(function(){
$('#tt').datagrid({
url: 'datagrid_data.json',
title: 'DataGrid - ContextMenu',
width: 600,
height: 300,
fitColumns: true,
columns:[[
{field:'itemid',title:"Item ID11s",width:80},
{field:'productid',title:'Product ID',width:100},
{field:'listprice',title:'List Price',width:80,align:'right'},
{field:'unitcost',title:'Unit Cost',width:80,align:'right'},
{field:'attr1',title:'Attribute',width:150},
{field:'status',title:'Status',width:60,align:'center'}
]]
});
});
</script>