Get on the SamTrans BART shuttle bus to the Colma station...then take BART to the Lake Merritt station (need to get on a Fremont-bound train).
From there, take an AC Transit bus to Jack London Square. Not sure of the exact route, but Transit Info (817-1717 from any Bay Area area code) can help out.
Allow about 1.5-2 hrs. for each way.
A day in downtown SF is much easier: just take the SamTrans express bus (KX) downtown. Only 1/2 hr.