Just a sample for Krish on the MonoDroid mailing list.

Activity1.cs

using System;
using System.IO;
using System.Text;
using System.Xml;
using Android.App;
using Android.Widget;
using Android.OS;

namespace MonoDroid.ReadWriteXML
{
    [Activity(Label = "ReadWriteXML Test", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        private EditText _id, _name, _description;
        private TextView _xmlView;
        private Button _save, _load;
        private readonly string _path = Path.Combine(System.Environment.GetFolderPath(System.Environment.SpecialFolder.MyDocuments), "Empp.xml");

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            _id = FindViewById<EditText>(Resource.Id.userId);
            _name = FindViewById<EditText>(Resource.Id.name);
            _description = FindViewById<EditText>(Resource.Id.description);

            _xmlView = FindViewById<TextView>(Resource.Id.xmlview);

            _save = FindViewById<Button>(Resource.Id.saveButton);
            _load = FindViewById<Button>(Resource.Id.retrieveButton);

            _save.Click += SaveOnClick;
            _load.Click += LoadOnClick;
        }

        private void LoadOnClick(object sender, EventArgs eventArgs)
        {
            var doc = new XmlDocument();
            doc.Load(_path);
            XmlNode root = doc.DocumentElement;
            var sb = new StringBuilder();
            var nodeList = root.SelectNodes("Employee");
            foreach (XmlNode node in nodeList)
            {
                sb.Append(string.Format("ID: {0} ", node.SelectSingleNode("Id").InnerText));
                sb.Append(string.Format("Name: {0} ", node.SelectSingleNode("Name").InnerText));
                sb.Append(string.Format("Description: {0} ", node.SelectSingleNode("Description").InnerText));
            }
            _xmlView.Text = sb.ToString();
        }

        private void SaveOnClick(object sender, EventArgs eventArgs)
        {
            var doc = new XmlDocument();

            if (File.Exists(_path))
                doc.Load(_path);
            else
                doc.InnerXml = "<Employees></Employees>";

            var newElem = doc.CreateElement("Employee");
            newElem.InnerXml = "<Id></Id><Name></Name><Description></Description>";
            newElem["Id"].InnerText = _id.Text;
            newElem["Name"].InnerText = _name.Text;
            newElem["Description"].InnerText = _description.Text;

            doc.DocumentElement.SelectNodes("/Employees")[0].AppendChild(newElem);
            doc.PreserveWhitespace = true;

            using (var wrtr = new XmlTextWriter(_path, Encoding.Unicode))
            {
                doc.WriteTo(wrtr);
            }

            _id.Text = _name.Text = _description.Text = "";
        }
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="internalOnly" package="monodroid.readwritexml" android:versionCode="1" android:versionName="1.0">
  <uses-sdk android:targetSdkVersion="14" />
  <application android:label="ReadWrite XML">
  </application>
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
  <uses-permission android:name="android.permission.WRITE_OWNER_DATA" />
</manifest>

Main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <TextView
        android:text="Add User"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textView1" />
    <EditText
        android:inputType="number"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/userId"
        android:hint="ID" />
    <EditText
        android:inputType="textPersonName"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/name"
        android:hint="Name" />
    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/description"
        android:hint="Description" />
    <Button
        android:text="Save User"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/saveButton" />
    <Button
        android:text="Retrieve Users"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/retrieveButton" />
    <TextView
        android:text="Text"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/xmlview" />
</LinearLayout>