To get into the Kratuga Ruins you have to enter the Ancient Hasrah Ruins first. The path will be shorter, if you'll use the second hidden entrance. It's well hidden between the rocks. Check the Screenshots tab for the location and the view of the entrance.
You can also use the Monster Zone Info from the main menu of the game (Esc->F5->4), find the Kratuga Ruins there and click the pathfinding button, which lead you to the hidden entrance.

Once inside, just go straight forward down the corridor until you'll find a Sealed Door on the left. To open it you'll need to either kill the monsters in Ancient Hasrah Ruins until they drop 3 pieces of Sealed Ruins Slate, which combine to Unsealed Ruins Slate, or just buy Black Spirit's Claw Piece the market, which is much faster.

Both Unsealed Ruins Slate and Black Spirit's Claw Piece are consumed once you enter the Kratuga Ruins. The monsters inside are tougher than in Ancient Hasrah Ruins.