Instructions
Complete the fifth "Story Mode" chapter of "Movie Two," in which you must use Indiana Jones and Mario Ravenwood to stand on two switches to defeat Belloq after opening the Ark of the Covenant. You will return to the main "Movie Two" level hub.
Go to your right when facing the city gate to find Colonel Dietrich standing under a red banner near a boat in the water.
Press "A" when standing next to Colonel Dietrich to speak with him. A prompt will appear on the screen offering to unlock his character for 75,000 LEGO pieces. Press "A" again to accept and you will now have control of Colonel Dietrich.
Destroy the nearby crate with the Colonel's explosives to reveal a hidden key, which you can then use on the crank beside the crate. The crank will open a hatch giving you access to the U-Boat 26 vessel. Press "A" to purchase the U-Boat for 30,000 LEGO pieces and pilot it into the bay on the southern part of the island to access the secret level.