Why can't I edit C# script although I have monodevelop installed? PLEASE HELP

I am making a game, I put the background and player; all fine, but when it comes to editing and double-clicking on the "script C#" named "player" it show me :"Unable to open Assets/Scripts/Player.cs:Check external application preferences. I read about this over the internet, but i guess the answers where for older versions as they said to install Monodevelop. I have the Unity3D 4.3.4 and Monodevelop comes built-in with it. I installed "Winmerge" as as revision control. I tried restarting Unity3D, my game, Winmerge.. but still the same thing. PLEASE GUYS HELP I NEED TO FINISH MY GAME BY TODAY AS I PROMISED MYSELF TO DO IT.