The system cannot locate the object specified. Error processing resource

I am getting an error message when executing the following code. The error is "The system cannot locate the object specified. Error processing resource 'http://localhost/witango/tutorial/tango.dtd'.
"

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE TAF SYSTEM "tango.dtd" >
<TAF Version="0x02000002">
<Comments />

<!--Main program-->
<Program>
<ActionRef Ref="Customer_Page" />
</Program>

<!--Program actions-->
<Actions>
<ResultAction ID="Customer_Page" Sig="null">
<ResultsOutput Ref="Customer_Page.Results" />
</ResultAction>
</Actions>

<!--Data sources-->
<DataSources />

<!--Output blocks-->
<Outputs>
<Output ID="Customer_Page.Results" Type="HTML">Welcome Customer!</Output>
</Outputs>
</TAF>

Does anyone know what is the right way to reference the dtd object?

Please Help!