Take a downtown #2 or 3 train to Fulton Street or Downtown A or C to Broadway-Nassau. For either at Fulton/Bway-Nassau, follow signs to the exit at Fulton & William Sts. Walk down Fulton St where it curves to the right. You will see the Seaport.
What Train Goes From Penn Station To South Street Seaport?
The best way is the subway.