There are generally taxi cabs available at the airport and at San Francisco hotels.  If you wish to take a taxi, the estimated rates are:

SFO to Mission Bay Conference Center: ~$30 - $35
SFO to Hilton Hotel (750 Kearny)*: ~$35 - $40

*Please note, there is more than one Hilton Hotel located in San Francisco, so be sure specify this one @ 750 Kearny.

Recommended cab companies: 

Yellow Cab: (415) 333-3333
http://www.yellowcabsf.com/

Desoto Cab. Co.: (415) 970-1300