Referring to the map below, the fastest way is to take I-680 North, exit at Marina Vista before going over the Bridge, and go west on Marina Vista.  Once on Marina Vista, you will go straight for a while, then wind up a small hill that passes throught the Shell Refinery complex, and then go down the hill towards a flat area.  The administration building will be off the first street that you can turn left at (beware of one way streets-- I believe that first street that you will encounter will be a one-way street to which you cannot enter.

You can also take highway 4, and take the Alhambra Avenue exit, going north. You will go through quite a bit of Martinez before hitting Escobar Street, at which you should turn right.