To reach him, you'll need to:
1. Go to the Goldenrod Department Store. It's the large building in the center of Goldenrod City.
2. Take the elevator to the fourth floor. You can't access the fourth floor until you've completed a specific event in the game.
3. Speak to the thief. He'll be standing in the corner of the floor.
After you confront him, he'll run away. He'll eventually lead you to a secret room where you can recover the machine part.