Hammer Arm is a Fighting-type move that is not in Rampardos's learnset in Pokémon Diamond. It can only learn it via breeding with a parent that knows the move.
Here are the ways Rampardos can learn Fighting-type moves in Pokémon Diamond:
* Level Up: Rampardos learns some Fighting-type moves by leveling up, such as Head Smash, Rock Slide, and Rock Tomb.
* TMs: Rampardos can learn some Fighting-type moves from TMs, such as Brick Break and Low Kick.
* Tutor: Rampardos can learn some Fighting-type moves from the Move Tutor, such as Close Combat.
If you want your Rampardos to learn Hammer Arm, you will need to breed it with a Pokémon that knows the move.