Wall cabinets screw into the wall only most of the time. If you have an island or peninsula then there are many options.
1. You can screw pieces of 2x4 to the floor, flush to the inside wall of the cabinet, put the cabinet over them and finish nail or screw through the cabinet sides or kick area into the 2x4's.
2. You could get long drywall style screws and just screw down through from inside the cabinet, through the cabinet floor, into the floor (easiest)
3. You could screw or nail from the outside, on an angle (similar to "toe-nailing"), assuming that the screws would be counter sunk so they won't stick out and that they will be covered by a baseboard that you put around the cabinets.
Sometimes I have had to use cabinets that didn't have a kick already built in so I just make a 2x4 on edge kick and screw it to the floor and screw the cabinet to it and then put baseboard.