In addition to the given solutions, you may also take a look at the following video guide for this short bonus level. Remember you need to grab the key from Indy's Office in order to unlock the bonus levels in the Artefact Room above the Main Hall.
After completing the level you will unlock two characters, "Young Indy" and "Boy Scout", beside the achievement, of course. You will also be able to buy "Fedora" in the Library, what is necessary to get the "You chose... wisely"-achievement for unlocking all the available characters in the game.
Credit for the video goes to http://www.gamerwalkthroughs.co.uk/