The flight out of San Pedro is just out to Municipal Airport and its there you will most likely meet the tour company and go by van/cab to the site.
I don't think you can go to the ATM and meet a guide, best to get an organized tour, too many obstacles if you don't.
might save a few bucks taking the water taxi to Belize city.
I'll bet Katie Valk can fix you up!
http://belize-trips.com/