You need to go to the tower where all the passed away pokemon are it is on the way to salaceon town and you go up to the very top and talk to two old ladies one of them will give yo HM04 (strength)
P.S you will need to have beaten the gym leader in canalave city to use it
HM04
you will need a Pokemon that know hm04 strength which you can get (diamond and pearl in the lost tower) and (platinum in iron island)
You will be able to use HM04 Strength outside of battle after you defeat the 6th gym which is in Canalave City.
HM04 strength is located in the Lost Tower in Pokemon Diamond. The Lost Towers is located on route 209 just south of Solaceon town.
north of route 209 the lost tower resides. on the top floor after battling trainers and wild Pokemon( I would bring dark Pokemon or bug Pokemon talk to a old ladie she will give you HM04
lost tower
In the top floor of the lost tower
HM04 Strength, can be found in Nimbasa City, specifically from a man in the building northwest of the Pokémon Center
in the cave with big boulder
HM04-Strength
stop cheating you noob
Nimbasa City, Top Left building