What exactly do you want to test?
If you are looking at getting data from/manipulating an xml file, you can add a reference like Microsoft XML v3.0 and then use the methods provided to carry out your tasks.
This can be done in 5.6
As far as my knowledge about testing XML goes, we either need to read from the XML file and check its output in our application under test or enter data in our application under test and check it against the output XML.
This can also be used from XAML testing(which is what I think you meant when you said WPF)
By adding the XML reference, you can get access to VBA's DOM parser which gives you the license to play around with the XML file.
You need to provide more details about WHAT exactly are you trying to test.
Thurst yourself into the wheel of madness and action in search of the center. Once you find the center, you cannot be moved.
Yes, there are no built-in functions, but there are rather good vba libraries available. So why not use the standard? The Microsoft XML lib TheVagabon mentioned is quite good. TP could not do better. It's just one click in menu Tools~References check the lib and you have all the functionality available.