Where is the guy that gives you rock smash soul silver?

You're likely thinking of the Move Tutor in Goldenrod City. He teaches you the move Rock Smash after you beat the Gym Leader, Jasmine.

Here's where to find him:

1. Goldenrod City: Go to the Goldenrod Department Store (the big building on the left side of the city).

2. Second Floor: Head up the escalator to the second floor.

3. Move Tutor: He's standing by himself in a small room on the right side of the floor.

Let me know if you have any other questions about SoulSilver!