In order to unlock the bonus levels you must first get 100% complete.
In order to do this, you must get true adventurer status on all levels, find all the parcels,find all the artifacts and unlock han solo. When you've done this, studs and hearts should fall from the ceiling. Then do the following...
1- Find the gem and place it in the artifact room
2- Go to Indy's office and find the key
3- Bring the key to the artifact room and turn it
4- The following levels should unlock- IN THIS ORDER...
In the artefact room, put the crystal on the post in the center. In another room, there are some puzzles to do, which reward you with a key that can be placed in the same post. Turn the key and some stuff happens. I think you have to collect treasure chests in the main levels to unlock.