This is not an Android issue but an XML issue. The XML element you are trying to reference contains a namespace i.e.
media. You need to import the XML namespace before you can reference it.
I'm not entirely sure how you achieve this in Java (I'm assuming you are doing this in Java) but in C# the code would be similar to the following.
XNamespace nsMedia = "media";
foreach (XElement xItem in xItems.Elements("item"))
{
XElement xContent = xItem.Element(nsMedia + "content");
XAttribute xUrl = xContent.Attribute("url");
string url = xUrl.Value;
}