TerrainToolkit API - TextureTerrain doesn't work in full build

Hi, I'm trying to create and texture a terrain at runtime using the TerrainToolkit API. If I use the script below and hit play in the Unity editor, the texturing seems to work fine, but when I build and run a Windows executable, the terrain gets generated but it's entirely white, no textures. What am I doing wrong?

 void Start()
     {
         var terrain = GameObject.Find("Terrain");
         var toolkit = terrain.GetComponent("TerrainToolkit") as TerrainToolkit;
         toolkit.PerlinGenerator(2, .5f, 9, 1);
 
         var cliffTexture = (Texture2D)Resources.LoadAssetAtPath("Assets/Standard Assets/Terrain Assets/Terrain Textures/Cliff (Layered Rock).psd", typeof(Texture2D));
         var sandTexture = (Texture2D)Resources.LoadAssetAtPath("Assets/Standard Assets/Terrain Assets/Terrain Textures/GoodDirt.psd", typeof(Texture2D));
         var grassTexture = (Texture2D)Resources.LoadAssetAtPath("Assets/Standard Assets/Terrain Assets/Terrain Textures/Grass (Hill).psd", typeof(Texture2D));
         var rockTexture = (Texture2D)Resources.LoadAssetAtPath("Assets/Standard Assets/Terrain Assets/Terrain Textures/Grass&Rock.psd", typeof(Texture2D));
 
         var slopeStops = new float[] { 20.0f, 50.0f };
         var heightStops = new float[] { .2f, .4f, .6f, .8f };
         var textures = new Texture2D[] { cliffTexture, sandTexture, grassTexture, rockTexture };
 
         toolkit.TextureTerrain(slopeStops, heightStops, textures);
     }