access xml element by attribute value

hoipolloi has given an XPath answer, which is fine - but I would personally use LINQ to XML. (See my blog post on code and data for reasons.)

var element = parent.Descendants()
                    .Where(x => (Guid?) x.Attribute("id") == id)
                    .FirstOrDefault();

This will perform appropriate GUID parsing on each id attribute (returning a "null" Guid? value for non-GUIDs). If you're certain of the text format of your ID, you can cast to string instead:

var element = parent.Descendants()
                    .Where(x => (string) x.Attribute("id") == idText)
                    .FirstOrDefault();

Change the FirstOrDefault to Single, SingleOrDefault or First depending on your requirements.

EDIT: It's not at all clear what's going wrong with the code you've posted. Here's a short but complete program which shows it working fine. Please compare this with your code:

using System;
using System.Linq;
using System.Xml.Linq;

class Test
{
    static void Main()
    {
        string xml = "<parent><foo id='bar' /><foo id='baz' /></parent>";
        XDocument doc = XDocument.Parse(xml);
        string idToFind = "bar";
        XElement selectedElement = doc.Descendants()
            .Where(x => (string) x.Attribute("id") == idToFind).FirstOrDefault();
        Console.WriteLine(selectedElement);
    }
}

You can use XPath to do this. For instance, the following matches all elements with an id of 'foo', irrespective of their location in the document:

//*[@id='foo']