You go to the "Museum of Olympus". Go over to Poseidon's statue. There will be a starfish on his face. Collect the starfish by jumping on it. Then go to his temple and enter. Finally, go to the altar, go to "Items", and press "USE" on the starfish.
His throne room is in the Temple to Poseidon, at the bottom of the lake. But you can only reach it when you have Hercules to move the boulder blocking the entrance. If you accidentally enter the Throne Room by slipping past the rock, you may not be able to get out again unless you have the Transporter Mirror.
In the realm of Poseidon is a beach and what should be a sea, but resembles a lake. The maze and the Hydra cave are below the surface, along with Poseidon's throne room.
To put the hydro scale on the altar in Mythology Island on Poptropica, you first need to obtain the hydro scale from Poseidon. To do this, solve the underwater labyrinth puzzle and find Poseidon's throne room. Once you have the hydro scale, return to the altar in the Grove of Temples and place it on the pedestal to activate the power of the gods.
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.
In Poptropica's Mythology Island, Hades' throne room is located in the Underworld, which is accessed through the entrance in the Tree of Immortality. Once you navigate through the challenges in the Underworld, you will reach Hades' throne room where you can defeat him and complete the island quest.
Place it on the offering stand in Hades throne room.
Hercules will help you reach the throne rooms of Poseidon and Hades. But you need the Touchscreen Mirror from Aphrodite's test in Poseidon's realm.
You can use the mirror by comical bubbles
On Mythology Island, you will meet the god Zeus, and his daughter Athena at the Tree of Immortality. You will find Aphrodite on the beach in the realm of Poseidon. The gods Poseidon and Hades are in their throne rooms, which you cannot enter until you have recovered the 5 sacred items and returned them to the Tree of Immortality.
Only Hercules has the strength to move the boulders at the throne rooms of Hades and Poseidon. And Herc will not help you until after Zeus has stolen the Sacred Items from you, in his bid to rule Poptropica. To reach the throne rooms with Hercules, you need the Touchscreem Mirror from Aphrodite's test (on the beach in Poseidon's realm).
She is at the Tree of Immortality. You must see her before you can access the throne rooms of Poseidon or Hades.
After you return the 5 sacred items to the Tree of Immortality, Hercules will help you move the rocks from the throne rooms of Poseidon and Hades. You will need the Touchscreen Mirror from the test by Aphrodite in Poseidon's realm.