function Start () {
    Terrain.activeTerrain.detailObjectDensity = 0.5;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Terrain.activeTerrain.detailObjectDensity = 0.5F;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        Terrain.activeTerrain.detailObjectDensity = 0.5F