(Warned, in this, there are several spoilers)
Once you have recovered the Five Sacred Items (pearl, hydra scale, minotaur ring, flower, and whisker), you cannot immediately enter the Throne Rooms, because it will take the strength of Hercules (and he won't help).
Instead, return to the Tree of Immortality where Athena will help you assemble a secret message on the items. When Zeus appears and steals them, Athena will send you back for Hercules, who will finally agree to help you reach the two Throne Rooms.
Hercules will help you reach both throne rooms, Poseidon and Hades. But he will not act until you have returned the 5 sacred items to the Tree of Immortality.
You will need the Touchscreen Mirror from Aphrodite's test on the beach in Poseidon's realm. The gods are Ares, Hermes, Aphrodite, Artemis, and Hestia.
-First, you need to get the mirror from Aphrodite in Posiden's realm.
-Then, you go to the other end of the lake in Hades's realm, and Hercules should destroy the boulder. Then go to Hades. He will then give you his crown.
-Then, in Posiden's realm, you go under water, and Hercules should stomp on the bridge/rock, and then you should be able to see Posiden. Then he will give you his staff.
-Then you go back to Zues, and Dodge his lighting bolts and such, while trying to shoot him down. Once you beat him, he'll give back the 5 items. Then (forgot her name) will say thank you, and give you the Island Medallion and 100 credits!
get the five sacred items, give them to Zeus, go to Hercules, and use the mirror to get there.
You cannot open it until you have the aid of Hercules, and he will not do so right away. Once you have all 5 sacred objects, you have to bring them to Athena at the Tree of Immortality, where their message can be revealed. Then they will be stolen by Zeus. Now Hercules will agree to help you, and he can move the boulders blocking you from Hades and from Poseidon.
Once Zeus has taken the 5 sacred items, Hercules will agree to help you, and will move the rocks blocking the way to Hades and to Poseidon. (You will need the Touchscreen Mirror from Aphrodite in Poseidon's realm.)
Hercules is not going to help you during the hunt for the Five Sacred Items. It is only after Zeus has taken them from you that he agrees. His contribution is to move the large rocks blocking you from the throne rooms of Hades and Poseidon, and to open the lock on the Olympus gate. You will need the Touchscreen Mirror from Aphrodite's test on the beach in Poseidon's realm. (The answers to her test are Ares, Hermes, Aphrodite, Artemis, and Hestia.)
After Zeus takes the 5 sacred items, you can get Hercules to help you reach the throne rooms of Hades and Poseidon. Once you have the crown and the trident, you can go to battle Zeus on Mount Olympus. Transport Hercules to the gate and he will smash open the padlock. (*before going to Olympus, you will need the drachma coin from cleaning Hades' temple)
After you collect all five sacred items, return to the Tree of Immortality. Zeus will take them. Then you can go to Hercules and talk to him. Use the Touchscreen Mirror to teleport to Hades throne room and Hercules will move the boulder.
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.
Hercules will roll it away for you. After Zeus obtains the five sacred items, Hercules will finally go with you, and will kick away the rock at Poseidon's throne room as well as removing the boulder at Hades.
To move the boulder on Mythology Island, you need to obtain the Golden Apple from Hades in the Underworld. Offer the Golden Apple to Atlas, who will then help you move the boulder so you can access the entrance to the Underworld.
To move the boulder at the throne room of Hades on Mythology Island in Poptropica, you need to use the trident that can be found in Poseidon's realm. Equip the trident and click on the boulder to move it aside.
You have to get Hercules to do it (I forgot how to get him to help, sorry!)
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.
Once Zeus has taken the five sacred items, Hercules will agree to help you. You will need the mirror from Aphrodite in the realm of Poseidon. Hercules doesn't like to walk far. In Poseidon's realm, Hercules will kick away the large rock so you can reach Poseidon's throne. When you have the Trident, go with Hercules to the realm of Hades, where he will move the huge boulder. Once you have the Hades Crown, transport back to the Olympus gate and Hercules will smash open the lock. (see related question)
get the five sacred items, give them to Zeus, go to Hercules, and use the mirror to get there.
You can only enter the throne room of Hades when you have Hercules with you. That cannot occur until after you have collected all five Sacred Items and had them stolen from you by Zeus. Then Hercules will get you in to see Hades, and also Poseidon.
Hercules will not help you until you have collected the 5 sacred items and had them stolen by Zeus. You will need the Touchscreen Mirror from Aphrodite's test on the beach. Transport yourself with Hercules to the Hades door and he will roll back the big rock. He will do the same for the rock blocking Poseidon's throne room.
You have to get Hercules to do it. He will only help you reach the throne rooms of Hades and Poseidon once you have taken the 5 sacred items back to the Tree of Immortality.