If you want to have Empoleon in your Pokemon Diamond game, you'll need to:
* Trade: Trade with someone who has Pokemon Pearl or Platinum and has caught a Piplup. They can evolve it into Empoleon and trade it to you.
* Use a Pokemon Bank: You can transfer a Piplup or Empoleon from Pokemon Pearl/Platinum to Pokemon Bank and then to Pokemon Diamond using Pokemon HOME.
Remember, you cannot find or catch Piplup or Empoleon in Pokemon Diamond.