You have to go to the power plant on route 10 first and talk to the guy by the machine and he tells you about the missing part. You find that by going in misty's gym in Cerulean City (but you HAVE to talk to the guy in the power plant first) in the gym you'll find a team rocket member and he runs out, chase him north of Cerulean and then he battles you and tells you where the power plant part is. After you get the part and return it, then return to Cerulean City and go north and at the end of route 25 is Misty with her boyfriend and she tells you to meet her at the gym.
