Where is Poseidon at the start of The Odyssey?
Book One, of Homer's The Odyssey, opens describing a council of the gods which is discussing why Odysseus is still missing (numerous years after the war). Zeus explains to Athena that Poseidon is angry at Odysseus for blinding Polyphemus (Poseidon's son). Prior to this though, Odysseus was kept on Calypso's island because she wished to marry him. Although all of the God's were fine with Odysseus returning, Poseidon did not wish this to be. Zeus, given that Poseidon is gone, decides that Odysseus can return.
Poseidon, at the opening of the text, is in the Ethiopians (or the end of the world). He had traveled there in order to oversee and accept a sacrifice of oxen and sheep. He was also partaking of a festival. Therefore, he was not at the meeting held at the the house of Zeus.