Main Menu Instructions?

You could have several different menus in the same scene using the GUI system - like this:

enum MenuID {MainMenu, HowToPlay} // menu types

var curMenu: MenuID = MenuID.MainMenu; // start at the main menu var playInstructions: String = "AD moves left/right\nWS moves forth/back";

function OnGUI(){ switch (curMenu){ case MenuID.MainMenu: // if main menu... // draw the main menu buttons: if (GUI.Button(Rect(...), "Start Game")){ // start the game } break; if (GUI.Button(Rect(...), "Quit")){ // quit the game } if (GUI.Button(Rect(...), "How To Play")){ curMenu = MenuID.HowToPlay; // select the "How To Play" menu } break; case MenuID.HowToPlay: // if how to play selected... // display the instructions: GUI.Label(Rect(...), playInstructions)); if (GUI.Button(Rect(...), "Main Menu")){ curMenu = MenuID.MainMenu; } break; } } You can use textures instead of strings with GUI.Label, producing much better results than plain text.

In case you choose to open an external file, you may try the code found in this question here: http://answers.unity3d.com/questions/16675/running-an-external-exe-file-from-unity.html

Still, to simplify things, consider that you put your text file inside the Assets folder and test this code to see if it meets your objective. This is in javascript by the way.

 import System.Diagnostics;
 function Start(){
  var stringPath = Application.dataPath+"/HowToPlayDoc.txt";
  var myProcess = new Process();
  myProcess.StartInfo.FileName = "Notepad.exe";
  myProcess.StartInfo.Arguments = stringPath;
  myProcess.Start();
 }

As you can see, it is calling the Notepad.exe and a .txt file. This should consider the platform the build was made for. There are more advanced methods to load data like this, but this is very simple, so check your needs and/or dig further into this solution if you find it necessary.