how to make a level load after hitting a score

Like this:

Create a score script - let's call it Score.js

   static var score : int;
   var nextLevelScore : int;
    var nextLevelName : string;

    function Update() {
          if(score > nextLevelScore) {
              Application.loadLevel(nextLevelName);
          }
     }

In each level attach this script to a game object and use the inspector to set the values.

Add to the score using Score.score e.g.:

  Score.score += 10; // add 10 to score

   Score.score = 0;  // initialize the score at the start of the game