Its probably glued to the but plate. You'll have to cut it off, remove the left over rubber with sandpaper/file or what ever works without too much damage to the plate, then glue a new one on. That's if I understand your question. If your are just trying to remove the pad to paint or whatever then there should be a couple of holes in the pad that either a torx or allen wrench will fit into to remove the screws.
Most of McMillan pads I have seen are epoxied on unless someone specified they wanted a screw-on pad.
The best thing for removal is to use is a screwdriver and a belt sander.
First, tear of the rubberized portion of the pad off so it doesn't clog the sandpaper and make a huge smelly mess. Place the remaining hard plastic butt base against the belt sander squarely. You will eventually sand down to the metal butt pad support frame. Once there, take a screwdriver and pry the support frame out. Once at this stage, go slowly, making sure you're carefeul to be square and even when sanding to where the pad base meets the stock. The hard part is keeping the butt stock perpendicular to the belt sander. Doing it this way will keep you from changing your length of pull and prevent chipping the gel coat of the stock.