I think that you need to change you script to:

    var passwordToEdit : String = "My Password";    
    var correctPassword : String = "OneTowTreeFour";
    var levelToLoad : int = 0;

    function OnGUI () {
        passwordToEdit = GUI.PasswordField (Rect (10, 10, 200, 20), passwordToEdit, "*"[0], 25);

        if(passwordToEdit == correctPassword) {    
            Application.LoadLevel(levelToLoad);    
        }    
    }

Besides rearranging your code, I changed your "LevelTovLoad" to "levelToLoad" and made it an intager instead of a string, so that you only have to put in one number. I changed the Private Variable "OneTowTreeFour" to a Public Variable called "correctPassword" and made OneTowTreeFour into the answer...

Oh, one more thing, if you want some of those really good "coder-people" to answer (or even read) your question, you should use descriptive questions, and try your best to explain your problem... Anyway, I hope I helped!

        var desiredUsername = "user";
        var stringToEdit = "";
        var desiredPassword = "admin";
        var passwordToEdit = "";

      function OnGUI () {
   // Make a multiline text area that modifies stringToEdit.
      GUI.Label(Rect (10, 10, 60, 20), "Username: ");
      GUI.Label(Rect (10, 35, 60, 20), "Password: ");

     stringToEdit = GUI.TextField (Rect (75, 10, 200, 20), stringToEdit, 200);

     passwordToEdit = GUI.PasswordField (Rect (75, 35, 200, 20), passwordToEdit, "*"[0], 25);

    if(stringToEdit == desiredUsername)
          {
    if(passwordToEdit == desiredPassword)
          {
        Application.LoadLevel(LevelTovLoad);
          }
          }
          }

       private var correctLogin : boolean;

       function Update()
              {
       correctLogin = (username == "user" && password == "admin");
              }


        function OnGUI()
           {
      var windowRect : Rect;
      windowRect.x = Screen.width / 2 - 100;
      windowRect.y = Screen.height / 2 - 50;
      windowRect.width = 200;
      windowRect.height = 100;

    GUI.Window(0, windowRect, OnWindowGUI, "Authentication");
          }

   function OnWindowGUI()
          {
     username = GUILayout.TextField(username);
     password = GUILayout.PasswordField(password, '*'[0]);
     GUI.color = btnColor;
   if (GUILayout.Button("Login") && correctLogin)
          {
       Application.LoadLevel(LevelTovLoad);

          }

          }