plz tell me
The name rater is in Slateport city to the northwest of the Pokemon Center (The house below and to the left a bit of the contest spectacular hall) His house has a sign that should tell you that his house is the name raters house. Remember, you cannot nickname Pokemon that do you not have your O.T. (Official trainer) ID or Pokemon that you got from events.
Hope I helped!
He is located in Slateport city west of the Pokemon Center. There is a sign next to his house.
Source: I do alot of naming