The move deleter in Pokemon Diamond,Pearl or Platinum is in Canalave City in the blue house above the Pokemart.
Where Is The Move Deleter In Pokemon Platinum?
Go to Canalave City and right below the pokemon center to the left would be a house, that would be the move deleter's house
In canalave city the blue roof house.
You see him in canalave city
canalave city
In canalave city underneath the gym in the blue roof house just above the pokemart