If you're in front of the pokemon center, go up to the street above it. There are two dark houses, go to the one all the way to the left. When u enter, talk to the boy at the top left of the room in the house and he will give you HM04 Strength. BTW i changed this answer to be more specific. The person before did helped out a lot to help me find the move strength.
In order to get HM Strength in Pokemon White, do the following:
1.Go to Nimbasa City.
2.Go to the Pokemon Center.
3.There is a building above the Pokemon center, but slightly to the left.
4.Go in the Building.
5.Yhere is a man in the back near the left. He will give you HM Strength.
Hope it Helped!
-MrEvan340
First you need to go to Nimbasa City then go to the house diagonally left behind the Pokemon Centre and talk to the Boy with Green/Blue hair and he will give you it.
Route 18
The rocks do not move until you have encountered Cobalion.
once you earn a certain amount of gym badges you will be able to teach a special move to your Pokemon( which will show up in HMS and tms part of bag).
By using the HM Strength.
i am trying to find the same thing. I have found that Luxray can move them. I have 2 Pokemon that learned the strength and it still wont work so i am going to try to make shinx evolve to Luxray. I recommend trying that if you don't have shinx.
It's actually called a hidden machine, and it is th move strength. Hope I helped :)
furret
You can move that boulder by using the hidden move strength.
Route 18
You get it from a house in Nimbasa City.
Strength can be found in the Top Left Building in Nimbasa.
The move in Pokémon Diamond that allows you to move boulders that are in the way as you're traveling the Sinnoh region is called Strength. Strength is a Hidden Machine that you can find in the top floor of the Lost Tower.
HM04-Strength
Lots of Pokemon push away stones.... but they need to know the HM (hidden move) STRENGTH.
you can get it from a guy called riley in front of the entrance to iron island
in nimbasa city in a house near the pokemon center.
In a house on the western side of Raimon City.