How do you get oricalcum plus from 100 acre woods in kingdom hearts 2

The Orichalcum+ isn't in a chest, you get it as a reward for clearing the Hundred Acre woods along with a cure element. For your reference, the other Orichalcum+ are found in the following ways.

- Clear the last level of Atlantica (Little mermaid)
- Win the Goddess of Fate Cup (Hercules)
- Receive as Gift from Moogles for Collection at least 1 of every synthesis material.
- Chest in the Central Computer Mesa (Tron)
- Chest in Sunset Terrace (Twilight Town)
- Chest in the Brink of Despair (The World that Never Was)