From South Bay via 280 or 101: Take 280 all the way to the end, or take 101 to the interchange and then take 280 NORTH to the end. 280 exits onto King St, and King St. ends at the Embarcadero. Bear left on the Embarcadero, and go about 1 mile. The Ferry Building is the gray-white thing with the pointy clock tower, at Market St.
From the North Bay via GG Bridge: After the toll plaza, avoid the right and left exits, and follow the main flow of traffic onto Lombard St. Stay on Lombard as long as it's multi-lane (about 14 blocks, a little over a mile). At the big intersection at Van Ness Av., turn left onto Van Ness. Go three blocks, then turn right on Bay St. In about 1 mile, Bay ends at the Embarcadero. Turn right on the Embarcadero and go about 1 mile. The Ferry Building is at Market St; it has a pointy clock tower.
From the East Bay via 80: Take the first SF exit off the Bay Bridge (Fremont St.). Turn left on Fremont and then right at the second light on Mission. Mission ends at the Embarcadero, and the Ferry Building is one block to your left, at Market St.
Parking: A terrible mess. Pay lots are available next to the Ferry Building. Street parking is more likely south of market than north. Pay attention to parking signs! Hints from regular commuters are welcome and will be posted here. In the meantime, consider taking MUNI or riding a bike or something. Bicycle parking is urban, with attendant risks; regulars suggest Steuart St. near the YMCA, where there's a lot of foot traffic.
Via MUNI: All inbound MUNI underground lines lead to the Embarcadero. Get off at the Embarcadero. Exit the station, and look for the gray-white building with the pointy clock tower at the end of Market St. That's the Ferry Building.
Via BART: Get off at the Embarcadero. Exit the station, and look for the gray-white building with the pointy clock tower at the end of Market St. That's the Ferry Building.