Pokémon Diamond and Pearl/Route 204 & Ravaged Path

Route 204
At this point in the game, this is a small problem and is quickly over with. A Pokémon with Rock Smash is needed (and a Oreburg Gym badge to use it ouside of battle). From Jublife, you should go north, through some grass, fighting the trainers if you want to, and should go into the cave.

Ravaged Path
From here, all you need to do is go to the right and use Rock Smash on one of the rocks and exit through the newly accesible doorway. You may also go to the left and smash some more rocks and collect an item (TM 39 Rock Tomb).

Route 204
From your position after leaving the Ravaged Path, go a couple steps west and then north on to Floamora Town.