Попробуйте использовать Xml Linq:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;
namespace ConsoleApplication51
{
class Program
{
const string FILENAME = @"c:\temp\test.xml";
static void Main(string[] args)
{
XDocument doc = XDocument.Load(FILENAME);
foreach (XElement add in doc.Descendants("add"))
{
string[] values = add.Attribute("value").Value.Split(new char[] {'.'});
values[values.Length - 1] = (int.Parse(values[values.Length - 1]) + 1).ToString();
add.SetAttributeValue("value", string.Join(".", values));
}
}
}
}