You can use an XPath query. Based on the structure of your document, this should snag you the precise value you are looking for:
var xml = XDocument.Load(@"c:\path\to\file.xml");
var nsManager = new XmlNamespaceManager(new NameTable());
nsManager.AddNamespace(
"oms",
"http://www.ibm.com/software/analytics/spss/xml/oms");
var cell = xml.XPathSelectElement(
"oms:outputTree/oms:command[@text='Regression']/" +
"oms:heading[@text='uid = 1015984.00']/" +
"oms:pivotTable[@text='Coefficients']/" +
"oms:dimension/oms:category/oms:dimension/oms:category/" +
"oms:dimension/oms:group/oms:category/oms:cell",
nsManager);
var number = (decimal)cell.Attribute("number");
Console.WriteLine(number);
Tweak as necessary. You will need these imports:
using System.Xml.Linq;
using System.Xml.XPath;