1. Defeat the Eterna Gym Leader: This is the fourth gym leader you'll encounter in the game. You'll find him in Eterna City, and he specializes in Grass-type Pokemon.
2. Speak to the Old Man: After defeating the Gym Leader, you'll find an old man sitting near the entrance of the gym. Speak to him and he'll tell you about the HM Move Rock Climb.
3. Head to the Valley Windworks: Follow the old man's instructions and travel to the Valley Windworks, which is located north of Eterna City.
4. Enter the Windworks: Inside the Windworks, you'll find a room with a large fan. Interact with the fan and you'll trigger a cutscene.
5. Get the HM: After the cutscene, you'll automatically be given the HM Move Rock Climb.
Note: You'll need a Pokemon that can learn HM moves in order to use Rock Climb in battle. You can use the Move Tutor to teach Rock Climb to any compatible Pokemon.
This should help you find Rock Climb in Pokemon Pearl! Happy adventuring!