The good news is you can ride MTA bus route 220 from inside LAX and up Robertson. There should be a stop along Robertson close to where you are going.
The bad news is that the bus runs only once an hour form about 7 A. M. to 7 P. M. If everything goes well, your ride should take about 60 minutes.
The alternative public transit option would be to talk an Airport Shuttle Bus to the LAX Transit Center and then ride Santa Monica Transit "Big Blue Bus" number 3 into downtown Santa Monica; there transfer onto a MTA number 20 or the MTA number 720 Rapid Bus along Wilshire.
For the MTA maps and schedules go to
http://www.mta.net, and for the Santa Monica Transit maps and schedules go to
http://www.bigbluebus.com/home.
The unfortunate thing about public transit in Los Angeles County is that if you are not taking a trip along a major bus corridor or a metro train line, transit rides can be long and difficult. This trip should really be easier on public transit. However, since your entire trip is not along a major transit corridor, it can take a while to reach your destination. If you must transfer, it can be even worse.
Good luck and have a nice stay.