So basically all you need is to create a document instance passing your XML string as an argument, and then you'll be able to navigate/process the XML content using E4X syntax.
I guess you refer to PowerShellRemotePSObject.getXml() function. If that is the case there are few utility classes comming with the plugin wich can help you to work wit result from powershell plugin.
There is sample workflow demonstrating how to extract values from .ps script output distributed with the plugin itself "Library/PowerShell/Samples/List directory content"
I usually use getXml()/getJson methods to get general knowledge of the structure of the returned result (what kind of objects are returned arrays,maps,simple types, complex objects and available properties ) . This info can also be found by looking at power-shell help .
Once I know what kind of objects are returned by the script you can use PowerShellRemotePSObject.getRootObject() and
b) if it is simple type (string/int ..) it is mapped to js simple type
c) if it is a complex object then use .getProperty() to access particular object properties
If you post some example xml returned I can provide some examples for extracting values from it.
Hope it helps!