How To Save Load to Hard Disc Drive C#

using Unity 4.0.1,

Windows 7

C#

I've found this but when I give in first code I start getting errors

 Assets/Scripts/Class/SaveData.cs(13,18): error CS0535: `SaveData' does not implement interface member `System.Runtime.Serialization.ISerializable.GetObjectData(System.Runtime.Serialization.SerializationInfo, System.Runtime.Serialization.StreamingContext)'

than if I modify it a bit - following his path don't know why that error go away but still...

 using UnityEngine;
 using System.Collections;
 
 using System.Text;
 //using http://System.IO;
 using System.Runtime.Serialization.Formatters.Binary;
 
 using System;
 using System.Runtime.Serialization;
 using System.Reflection;
 
 
 
 [Serializable ()]
 public class SaveData : ISerializable {
 public bool foundGem1 = false;
 public float score = 42;
 public int levelReached = 3;
 
     public SaveData (SerializationInfo info, StreamingContext ctxt){
         //Get the values from info and assign them to the appropriate properties
         foundGem1 = (bool)info.GetValue("foundGem1", typeof(bool));
         score = (float)info.GetValue("score", typeof(float));
         
         levelReached = (int)info.GetValue("levelReached", typeof(int));
     }
     
     //Serialization function.
     
     public void GetObjectData (SerializationInfo info, StreamingContext ctxt)
     {
     
         info.AddValue("foundGem1", (foundGem1));
         info.AddValue("score", score);
         info.AddValue("levelReached", levelReached);
     }
     public void Save () {
 
         SaveData data = new SaveData ();
     
         Stream stream = File.Open("MySavedGame.game", FileMode.Create);
         BinaryFormatter bformatter = new BinaryFormatter();
         bformatter.Binder = new VersionDeserializationBinder();
         Debug.Log ("Writing Information");
         bformatter.Serialize(stream, data);
         stream.Close();
     }
 
     public void Load () {
 
         SaveData data = new SaveData ();
         Stream stream = File.Open("MySavedGame.gamed", FileMode.Open);
         BinaryFormatter bformatter = new BinaryFormatter();
         bformatter.Binder = new VersionDeserializationBinder();
         Debug.Log ("Reading Data");
         data = (SaveData)bformatter.Deserialize(stream);
         stream.Close();
     }
     
 public sealed class VersionDeserializationBinder : SerializationBinder{
         public override Type BindToType( string assemblyName, string typeName ){
             if ( !string.IsNullOrEmpty( assemblyName ) && !string.IsNullOrEmpty( typeName ) ){
                 Type typeToDeserialize = null;
                 
                 assemblyName = Assembly.GetExecutingAssembly().FullName;
                 // The following line of code returns the type.
                 typeToDeserialize = Type.GetType( String.Format( "{0}, {1}", typeName, assemblyName ) );
                 return typeToDeserialize;
             }
             return null;
         }
     }
 }
 
 

and now I'm getting errors:

 Assets/Scripts/Class/SaveData.cs(41,17): error CS0246: The type or namespace name `Stream' could not be found. Are you missing a using directive or an assembly reference?

and for the last class inside class I thought it would be wrong but it seems it's not as I get no errors so I'm even more confused as it seems I did steps correctly

I basically puted all in on bottom of each

I really don't understand 5% of this code

is it really necessary to write 3 pages just to save 1 int on HDD if not can someone show me simpler code how to save data on HDD so I could read write on it