The pack replaces the standard 
diamond horse armor with a matching pink design. There is no new item to find. Any diamond horse armor you already have becomes the pink version once the pack is active.
Equip the armor on your horse
1. Tame a horse and put a saddle on it.
2. Open the horse's inventory. Either sneak and right-click the horse, or mount it and press your inventory key.
3. Place the diamond horse armor in the armor slot.
4. Close the menu and step back, or press F5 while riding, to see the pink armor on the horse.
If you don't own diamond horse armor yet, you can search for it in loot chests in survival. In Creative, you can take it straight from the inventory.
Match the rest of your gear
- Wear your full
diamond armor set so you and your mount share the same pink look. - Nautilus armor is also reskinned, and its item icon is changed to fit the theme. Equip it the normal vanilla way on whatever wears it in your version, and it will match your other pink gear.
- Horse armor keeps its usual shape, so you can still tell it apart at a glance in your inventory.
If the horse armor still looks blue, check that no other resource pack above this one changes horse armor.