Good Day Geraint,
GetDOM() gets the entire DOM for a specific data source. The parameter should be the name of the data source. In the Data Source taskpane, what is the name displayed there? This is what should be passed into the GetDOM() method.
Once you have the dom, then you can query it for the PO node. I hope this helps, good luck!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge