You can use a XPath query to locate the element based on the id. For example something like:
System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument();
xmlDoc.LoadXml(@"
<DataDownload>
<ClusterId id=""00 15"">
<attrib>00 00</attrib>
<attrib>01 00</attrib>
<attrib>02 00</attrib>
<attrib>03 00</attrib>
</ClusterId>
<ClusterId id=""00 00"">
<attrib>00 00</attrib>
<attrib>04 00</attrib>
<attrib>05 00</attrib>
<attrib>07 00</attrib>
</ClusterId>
<ClusterId id=""07 02"">
<attrib>00 00</attrib>
<attrib>00 02</attrib>
<attrib>00 03</attrib>
<attrib>03 03</attrib>
<attrib>06 03</attrib>
</ClusterId>
</DataDownload >");
System.Xml.XmlNodeList result = xmlDoc.SelectNodes("//ClusterId[@id='00 15']");
if (result.Count > 0) {
}