you didn't set xml reader in your grid like.Grid need to expect xml
xmlReader: { repeatitems: false, root:"TradingResults",row:"TradingResult" }
and also set xmlmap
for each column like
{ name: 'Instrument', index: 'Instrument', width: 55,xmlmap:"Instrument" },
Also did you include the DnR lib which is
<script src="grid/js/jqDnR.js" type="text/javascript">
</script>
This one is important
Update: Since this answer is getting much attention, here people may find additional information on jqgrid with xml data.