Keeping a global score

Unity Engine
1

Hi,
I’m trying to keep a global score with my game. all the AI in the game that you need to kill have a script on them called “Take_Damage”
this script controls all thier death effects and removing the game object.
To keep a score i tried adding

scoreKeeper.SendMessage("IncrementScore",1);

where scoreKeeper is a GameObject variable

but for some reason i’m unable to drag my scoreKeeper object from the scene into the variable slot on the script, the slow will only allow me to insert prefabs!
but inserting a prefab there is useless, it just creates a new score keeper and adds 1 point to it, never getting anywhere.
what should i do?

2 
ScoreKeeper.js

static var score : int = 0;

Something.js

function AddScore() {
  ScoreKeeper.score += 4010325;
}