Because Odysseus is not there to take someone/thing back with him and is only there for information, he does not have to enter the land of the dead through Cerberus' station at the River Styx. Cerberus was there to keep the living out of the land of the dead. Instead of entering in that spot, Circe has his men enter through a different route:
She tells Odysseus to travel to the shore of Oceanus. Once there, he is to go "near the place where the rivers Pyriphlegethon and Cocytus (which is a branch of the river Styx) flow into Acheron."
She then tells him "you will see a rock near it, just where the two roaring rivers run into one another."
That is the location of his entrance. There are several rites he must perform before speaking to Tieresias. Never does Odysseus encounter Cerberus.
Yes Odysseus encountered Cerberus because he had to talk to one of the people down in the underworld.