Instructions
Catching Lotad
Load "Diamond," "Pearl" or "Platinum."
Travel to Route 204, which is south of Floaroma City. Route 204 has a northern and southern part, divided by a cave. Remain in the northern part.
Walk in the grass until you encounter a Lotad. Attack it with weak moves until its health meter is yellow or red.
Throw Pokeballs at it until you capture it.
Save the game and turn off your Game Boy.
Importing Lotad to "HeartGold"
Insert "Diamond," "Pearl" or "Platinum" into the Game Boy slot on your Nintendo DS. Turn on the DS.
Select "Migrate" from the main menu. Choose the Lotad and any other Pokemon you want to import.
Load your "HeartGold" game and travel to Pal Park, which is immediately north of Fuchsia City.
Walk to the northeastern end of Pal Park. You'll find a pond there. Walk around the perimeter of the pond until you encounter the Lotad.
Weaken it and then throw Pokeballs to catch the Lotad. It's now a part of your "HeartGold" collection.