How do you get strength to work in Pokemon pearl?

Answer 1

To use strenght outside battle you need:

A Pokemon that knows strenght or the HM so you can teach it to a Pokemon

The Mine Badge from Byron in Canalave City.

Have a Pokemon knowing strenght on your team and go to a place where you need strenght. Press A in front of the big, grey boulder you want to move and use strenght. Now when you walk, you'll push the boulder in front of you.

You don't have to use strenght for every new boulder, you'll only have to re-use it if you go in another room.

Answer 2

I think you have to beat the canalave gym leader