You can enter the realm of Poseidon from his temple at the far right of the island. Use the starfish (from Poseidon's statue in the museum) as an offering on the altar.
To get to the throne room (except as a glitch), you will need Hercules. Hercules will not help you until you have returned all of the 5 sacred items to the Tree of Immortality. Get the Touchscreen Mirror from Aphrodite on Poseidon's beach, so that you can transport Hercules to the realms of Poseidon and Hades (he does not like to walk).
Poseidon is found on Mythology Island, which is a Poptropica island.
From Poseidon's statue in the museum.
the pink starfish
In Poseidon's realm, you click on her.
It is in the museum, on the face of Poseidon's statue.
The only doors on Mythology Island are at the temples of Hades and Poseidon, and on the Minotaur's lair.
If you are talking about Mythology Island, there is no such thing there. Mythology Island only has three 'kingdoms': Hades, Poseidon, and Zeus.
Offer the starfish from the Poseidon statue in the Museum.
The offering for Poseidon is a starfish. There is one on the face of his statue in the Museum of Olympus.
zombie poptropica
all you need is Hercules's help
You need a starfish. There is one on Poseidon's statue in the museum on main street.