if you have a gap between the mounting tabs and the bottom of the counter, cut a 24"x1-1/2" strip of wood the gap. The thickness of the wood will need to be the same as the gap between the DW tabs and the counter. Use silicone to adhere the wood to the bottom of the granite with a bead the entire length of the strip. Hold it in place over night with a couple strips of masking tape. The following day you can remove the tape and screw the tabs into the strip just like any other wood based counter top. It would also be a good idea to finish the wood to match the cabinets.
If there's no gap then it is a different matter... Let us know if there's no gap.