A.) If you're asking where it is (meaning where HM04 is), it is in the Rusturf Tunnel between Verdanturf town and Route 116.
B.)If you're asking when you can use it, the answer is that you can only use strength once:
1). A pokemon in your party knows it
2). You have the Heat Badge from defeating Flannerly of lavaridge town.
3.) You are next to and facing a boulder that strength can be used on.
C.)If you mean where it can be used, you can use it whenever there's a boulder and the game says something like "it a big boulder, a pokemon may be able to move it" or something like that. you'll also need a pokemon that's learned it in your party.
use rock smash on the rocks in rusturf tunnel and a guy will give it to u
You can find a Fire Stone in the Fiery Path after you get the HM for Strength.
If you have beaten Mauville Gym and have gotten HM ROCK SMASH, go to RUSTURF TUNNEL and clear out the rocks. The digger will grant you HM STRENGTH.
It is Strength. You will get it A. From a gym leader or B. Look around Mauville.
All
H.M
You have to teach one of your Pokemon the HM 'Strength' and use it.
use rock smash on the rocks in rusturf tunnel and a guy will give it to u
the hm move strength is found in the cave Rusturf tunnel
strength after the chamber
In rusturf tunnel.
You can find a Fire Stone in the Fiery Path after you get the HM for Strength.
If you have beaten Mauville Gym and have gotten HM ROCK SMASH, go to RUSTURF TUNNEL and clear out the rocks. The digger will grant you HM STRENGTH.
It is Strength. You will get it A. From a gym leader or B. Look around Mauville.
All
Veilstone.
You need to have a Pokemon that knows the move Strength (HM 04). But if you have a Pokemon that can learn it but you only want to use the move once, be warned, once the Pokemon learns a HM move, it cannot be erased, or you might say, it cannot learn a move in replace of it.