Of all the hidden moves in Pokemon Brilliant Diamond and Brilliant Pearl, MT100 Rock Climb is one of the easiest to miss. This hidden move is actually quite well hidden and if you don't know where to look, you may end up not being able to scale those pesky cliffs. This is where you can find this important TM.
Before you can access this TM, you will first need to be able to use Rock Smash and Strength. Having Defog is useful for the route to where Rock Climb is, but it is not necessary. Far enough in the story, you will be given the task of going to the lakes because Team Galatic has no good intentions. The last lake you will need to go to is Acuity Lake.
Take the path through Mount Coronet going to Evisopolis or Celestic Town. After traveling for a while, the path will connect to Route 216 and then to Route 217. When you reach Route 217, a blizzard will appear and it will be difficult to see. Keep left after the blizzard begins until you find a cabin. Enter and the hiker will tell you that he dropped the TM containing Rock Climb.
Go back outside and behind the house and you will find an item. Collect it to get the TM Rock Climb. You will now be able to scale cliffs by walking towards them and pressing A. You will need to get the seventh gym badge before you can use Rock Climb outside of battle.