The xml returned by webservices invariably use namespaces. To access data contained in the xml you would need to declare namespace . To declare namespace you would select the process,right click the process select properties, click the advanced tab to create namespaces
For example the following is a example of namespace declared in workbench
where greg is the namespace prefix
I have included a simple process which makes a web service call to Amazon, using namespace I get data out of the response. The following is an example of using namespace in workbench
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Original article at http://eslifeline.wordpress.com/2009/02/06/using-namespace-in-workbench/.