Strength is one of the hidden moves you need to find in Pokemon Brilliant Diamond and Shining Pearl. This move allows you to push boulders away and clear paths. Before you can use this move, you will need to defeat the Gym Leader in Canalave City. This is the sixth gym you will face, but you will be able to get TM as soon as your second gym battle. Here's where to find this TM.
Before you can get this TM, you will first need to get to Hearthome City. By the time you reach this city, you will have fought against two gym leaders. After completing a few tasks in Hearthome, you will be assigned the task of going to Veilstone City. To get there, you'll need to take Route 209 which is east of Hearthome City.
Make sure you stop at the Pokecenter before heading out as you will have to face several trainers on your way. Follow the path until you find the Lost Tower. This landmark is near the end of Route 209 before entering Solaceon Town. Enter the tower and go upstairs. When you reach the top, you will find two elderly women in the center of the room. Talk to the lady on the left to get the TM. By talking to the lady on the right you will get the Cleansing Tag.