Instructions
Obtain five Oran Berries. The easiest way to find these berries is to fight and defeat wild Chuckle Pokemon. They can also be gained by defeating wild Pikachu, or on the level 10 Pikachu in the PokeWalker course.
Travel to Route 39 once you have five berries in your possession.
Make your way to the northernmost part of the Route. You will come across the Moomoo Farm as you walk north.
Enter the farm and move over to the sick Moomoo.
Press "A" to examine the Pokemon, then feed it your Oran Berries. After the fifth berry, the Moomoo will be cured and you will receive the Seal Case as a reward.