From SFWiki

To get to UC Davis through public transportation, take BART to the Richmond Station. Leave the BART platform and get onto the Amtrak platform (just walk down one set of stairs and up another... very easy). Buy a train ticket to the station at Davis, CA.

Once in the city of Davis, notice that most streets are named in an alphabet and numbers system. The train station is at 2nd and J St. Walk down to A St. and up to 3rd St. and you will find one corner of the UC Davis campus.