Instructions
Go into Indiana Jones' office at Barnett College. You must uncover five chests in this room to get the key.
Use the character Willie Scott, or throw an explosive to break the glass wall in the back right corner of the room to get the first chest.
Switch to a character with a high jumping ability -- most female characters will work -- to get the second chest on some shelves in the back of the room.
Use a scholarly character to decipher the glyph panel in the back left corner of the room. This will cause the panel to slide to the side and reveal the third chest.
Move the Thuggee Statue on the right side of the room, and use a small character such as Short Round to crawl into the vent and retrieve the fourth chest.
Get the fifth and final chest by using an explosive on the shiny discs in the front left part of the room. Dig here using a shovel to uncover the fifth chest.
Place all of the chests in the center of the room, and they will open to reveal numerous Lego blocks. Assemble these and smash the box that forms to get the key.
Head to the Artifact Room and place a blue diamond in the middle of the room on a pedestal -- this will reveal a crank. Put the key on the crank to open three slabs that provide access to the bonus missions.