How do you move the boulder at the throne room of Hades on Mythology Island?

You can't until you have returned the 5 sacred items to the Tree of Immortality. Then Hercules will agree to help you. The notes on the items say "whoever wields the five sacred items will rule all of Poptropica" and that's what Zeus wants. Athena tells you that Hercules will help you stop him.

You will need the Touchscreen Mirror from Aphrodite's test (Poseidon's realm) to take Hercules to the Poseidon and Hades throne rooms.