To get the Eyeball Frog, you must first go to Domaine Zora (play Zelda's lullaby to stop the waterfall).
If you don't have a blue flame (there are 300 rupees at the apothecary), go to the Zora fountain (behind the king, where Jabu Jabu was)
then jump from iceberg to iceberg to reach the ice cave.
to take a blue flame and return to King Zora.
Use your blue flame on him to burn the block of ice that traps him and give him the prescription.