Copied from the xbox 360 solution:
Travel to oasis to start of these quests at the fast travel station
An introductory cutscene will ensue promising new enemies, challenges, locations, and loot!
This is a story related achievement and cannot be missed as you progress through this DLC. With that being said, here is all the additional info you could possibly need.
You will need to complete the first 8 story missions before the "X Marks the Spot" mission becomes available. These are:
Chapter 1: A Warm Welcome
Chapter 2: My Life for a Sandskiff
Chapter 3: A Study in Scarlett
Chapter 4: Two Easy Pieces
Chapter 5: The Hermit
Chapter 6: Crazy About You
Chapter 7: Whoops
Chapter 8: Let There Be Light
Upon accepting the mission "X Marks the Spot", you will be told to rendezvous with Scarlett. Naturally, that won't last long as Scarlett's long awaited betrayal finally comes to light. Defeat the baddies that Scarlett sends your way and then follow your objective back to Wurmwater. Continue toward your objective until you find yourself at The Leviathan's Lair (whether you like it or not). I don't want to say much more for fear of some serious spoilers, but needless to say, this is Borderlands and you will be ending this DLC by fighting a giant monster. As usual, aim for the glowy spots. The achievement will unlock when you turn in the mission to Shade at Oasis. Good luck!
*HINT: Run across the dead monster to get to the secret loot room.