If there is any namespaces in your xml, you can not parse your xml string. I am not sure but OO9.02(with content patches 9_00_06) uses XPATH 1.0 version for XPATH evaluator. If you want to parse XML with namescapes first you should remove the namespaces from xml string using regular expression.
It does not matter if it's a config file or a return string from webservice. You can get all values if you know the structure(I mean paths).
I am waiting for the new patch for JSON format, so hopefully I will be parsing any XML string in OO without using workarounds.