How do you see the missing machine for the power plant on soul silver?

Head east from Cerulean city and go to the top right section of the grass area, the surf downwards on route ten, enter the power plant and then go and speak to the manager. Then when you're about to leave the security guard will anonce that there is a shady figure back at ceru;ean city. Go back and enter the gym where he will confront you but not fight you, when he leaves go out after him, go upwards on a gold bridge where he will hide in between two trainers, talk to him and beat him

Lvl 39 Golbat.

he then tells you the location of the hidden machine part and you can only get it after you defeat him!!!!!!!!!!!!!!!!!!

Hope that was helpful!!