Edit in JSFiddle

$('#tbl_viewalluser').dataTable({
    "bJQueryUI": true,
    "bSortClasses": false,
    "bAutoWidth": true,
    "bInfo": true,
    "sScrollY": "100%",
    "sScrollX": "100%",
    "bScrollCollapse": true,
    "sPaginationType": "full_numbers",
    "bRetrieve": true,
    "oLanguage": {
        "sSearch": "Search Anything:"
    },
    "bProcessing": true,
    "bServerSide": true,
    "sAjaxSource": "<?php echo site_url('backend/loadUser') ?>",
    "fnServerData": function(sSource, aoData, fnCallback) {
        $.ajax({
            "dataType": 'json',
            "type": "POST",
            "url": sSource,
            "data": aoData,
            "success": fnCallback
        });
    }
});

<table class="display table " id="tbl_viewalluser" > 
            <thead> 
                <tr> 
                    <th>Username</th> 
                    <th>Email</th> 
                    <th>Role</th> 
                    <th>Is Locked</th> 
                    <th>Last IP</th> 
                    <th>Last login</th>
                    <th>Created</th>
                    <th>Action</th>
                </tr> 
            </thead> 
            <tbody> 
            </tbody> 
</table>