Placing textures

In case when you have terrain you have to select option in the middle (brush icon) and set specified texture.

But if you want to make Minecraft clone, terrain is useless. Minecraft world is made from separate cubes and most efficient way to do this is from for loop.

Take a look:

 using UnityEngine;
 using System.Collections;
 
 public class BasicWorldGeneration : MonoBehaviour
 {
     public GameObject Block;
     public int AmountX;
     public int AmountY;
     public int AmountZ;
 
     void Awake ()
     {
         for (int x = 0; x < AmountX; x++)
         {
             for (int y = 0; y < AmountY; y++)
             {
                 for (int z = 0; z < AmountZ; z++)
                 {
                     Instantiate (Block, new Vector3 (x + 0.5f, y, z + 0.5f), Quaternion.identity);
                 }
             }
         }
     }
 }

This is a very basic world generation with configuration. You can select a block and adjust rows in all axes.

Browse series in Youtube. There are many tutorials how to make Minecraft in Unity.