There's no such thing specifically called the Poke Salon. The one that closest resembles a Poke Salon is the Salon in Join Avenue, but that only exists in B2W2.
Also in HGSS, there are the Haircut brothers & they'll groom a Pokemon & add to their happiness. They don't exist in Platinum.
For Platinum, the only thing close is the lady who gives massages; she resides in Veilstone City, in the house on the same platform & closest to the Gym.
A massage will only add +3 to your happiness quota.