What happens is that the dom object, being namespace aware, knows your nodes belong to your default namespace. But as you haven't declared that this namespace applies to nodes with a certain prefix you have no way of specifying which namespace the nodes you wish to select belong to. If you just use the node name in your selection then you will draw a blank; the dom object will assume you are asking for nodes with NO namespace.
The setProperty method allows you to assign a temporary prefix for the namespace and use it in your selection queries.
For more information about WHY these methods don't seem to work when there is a default namespace see JJR's FAQ: How to match nodes if there's a default namespace in the xml.