There is no cow level
Whimsyshire is a Diablo 3 secret level that provides a change of scenery from the usual areas you encounter in the game, here's a quick few things to know about the area before we begin the guide.
We need a Staff of Herding, which can be acquired by giving your Blacksmith 5 rare items along with the staff plans.
Plan: Staff of Herding
The plan is a rare drop from the boss Izual at "The Great Span" in Act 4. It is highly recommended that you fight this boss on Normal difficulty, although the plan can drop on any difficulty. After you have the plan it's time to search the world of Diablo 3 for the other 5 rare items we need.
The first item on our list is found in Act 1, on any floor of the Cathedral. It is recommended that you search only Cathedral Level 1 as it has the greatest chance for the Black Mushroom to spawn. The mushroom can spawn in any room. For an example of the Black Mushroom looks like, check the photo. Like all the other items, the Black Mushroom is not guaranteed to spawn in your game, meaning you will have to keep remaking games a few times until you find it.
This item is a random spawn in Leoric's Manor in Act 1. The fastest route to getting this item is by starting a new game on the quest "The Imprisoned Angel" and then taking the waypoint to Leoric's Manor. Unlike the Black Mushroom, Leoric's Shinbone always spawns in the same room making it much easier to find. If the fireplace has burnt logs in it, simply click them and you will find the shinbone. If no logs are present, it's time to make a new game and try again.
Wirt's Bell can be bought for the low price of 100,000 gold in Caldeum Bazaar in Act 2 from everyone's favorite vendor, Squirt the Peddler.
The Liquid Rainbow is found in Act 2 in a Mysterious Cave that can only be opened by a rare spawn merchant, who you must save from ghosts. After killing the ghosts and speaking to him, he will open the cave allowing you access. Once inside the cave you need to look for a Mysterious Chest that can only spawn on the first floor of the cave. Unfortunately even if the cave spawns in your game, there is a chance the Mysterious Chest will not. This is perhaps the hardest of all the items to get. The quickest way to find the merchant and his cave is to start a new game with any quest in Act 2 past the "Blood and Sand" and then take the waypoint, "Path to the Oasis". From there run to the right and follow the bottom edge of the map until you find a little path that leads to the cave.
The final item drops from a rare spawn monster named Chiltara in the Caverns of Frost level 2, which is found in the Fields of Slaughter in Act 3. Both the Caverns of Frost and the monster Chiltara are rare spawns, making this a very time consuming item to find. Start a new game on the quest "Machines of War" at the second part of the quest - "Destroy Ballistae" and then take your waypoint to the Bridge of Korsikk and head northwest exploring the area until you find the cave.
Note: Two caves can spawn in this area, Icefall Caves is not the right one, Chiltara can only spawn in Caverns of the Frost, level 2.
Finding the entrance
Once you have collected all the pieces its just a matter of crafting the Staff at the Blacksmith. Then head to Old Tristram Road at the start of Act 1. As long as you have the staff in your inventory the cow king will appear and have a brief conversation before opening the level. Note when entering the secret level it may be full of monsters near the entrance so be careful, especially on hardcore.