Reach Herodotos on the beach of Attika. Go by land, or by sea; he's on the southwestern side of the peninsula.

From there, head to the Pnyx in Athens. Athens is the main city in Attika, and extremely hard to miss. The Pnyx, however, is a little more elusive. Head to the southeastern side of the city. Look for a stage-like area with lots of people surrounding it. Perikles and Kleon are here, speaking to the masses.

Have a short conversation with Perikles, tell him you're looking to get to the symposium. He will then mention what must be done to get an invitation. Gather some information from him, and as the conversation wraps, the mission will be complete.