You must find R2-D2, C-3PO, Chewbacca, Princess Leia, and Luke Skywalker to unlock Han Solo. After snagging the idol in Free Play on the level The Lost Temple, blow up the barred door in the upper-right corner. Collect Treasure 6 and look for a container with his head sticking out. Smash the Container and it will free C-3PO and he will vanish and the numbers 1/5 will appear. In a Lost Temple cutscene, before Indy surrendered the idol to Belloq, he showed them his head!