You unlock Rock Smash very early but can’t use it at first. This guide on How To Use Rock Smash In Pokemon Brilliant Diamond And Shining Pearl will explain how the new HM (Hidden Machine) mechanics work so you can bust your way through rocks and remove them from your path.
You learn Rock Smash in one of the very early caves in Pokemon Brilliant Diamond And Shining Pearl, by speaking with a Hiker near the entrance, can’t miss it. He gives you both a TM for Rock Smash and the Hidden Moves app. The Hidden Move app allows you to learn all of the HM’s as you progress through the game. Unlike the earlier versions of the Pokemon games, these do not need to be taught to Pokemon in your party. When you use a Hidden Move it will summon a wild Pokemon to do the action for you. This means you don’t have to carry HM mules arounds and can just focus on the party and abilities you want them to have.
How To Use Rock Smash In Pokemon Brilliant Diamond And Shining Pearl
As soon as you learn Rock Smash you can find it inside the Poketch App that appears on your screen. However, you do not need to use this app to use Rock Smash. Also, teaching any of your Pokemon the TM Rock Smash is useless (in terms of actually smashing rocks, it’s still a decent move), you will need to progress before you can physically use Rock Smash.
To use Rock Smash, you simply approach a Rock that can be smashed and press the A button. If this doesn’t work, check your Hidden Moves app. It is very likely that the Rock Smash HM is grayed out. This means you have learned the ability but you do not yet have the experience to actually use it. You simply need to complete the first gym and obtain the badge. You will then be able to smash rocks like I explained previously.