TerrainTest.cs

using UnityEngine;
using System.Collections;

public class ClickTerrain : MonoBehaviour
{
	void Start()
	{
		var terrain = Terrain.activeTerrain;
		var td = terrain.terrainData;
		var resX = td.heightmapWidth;
		var resY = td.heightmapHeight;
		var strength = 1f;

		Vector3 clickPos = Vector3.zero;
		Ray ray = Camera.main.ViewportPointToRay(Vector3.forward);
		RaycastHit hit;

		if (Physics.Raycast(ray, out hit))
		{
			var pos = clickPos - terrain.GetPosition();
			pos.y = pos.z;
			pos.z = 0;

			pos = new Vector2(pos.x / td.size.x, pos.y / td.size.z);
			pos = new Vector2(pos.x * td.heightmapWidth, pos.z * td.heightmapHeight);

			float[,] heights = td.GetHeights(0, 0, resX, resY);

			//heights[(int)pos.x, (int)pos.y] = strength;
			for (int x = 0; x < resX; x++)
			{
				for (int y = 0; y < resY; y++)
				{
					heights[x, y] = Mathf.Clamp((heights[x, y] + 0.01f), 0f, 1f);
				}
			}

			td.SetHeights(0, 0, heights);

		}

	}
}