As mentioned above you have to grind or file the tool down to fit and turn clockwise to compress it. I had to use alot of cussing too when I changed out my rear brake pads while I did my axle bearings last week.
When I did mine, the pistons were frozen in the bores, so the tool was useless at first and I thought I was doing it wrong. Turns out, the pistons were frozen and I ended up popping them out of the calipers and hitting the bore and pistons with some emery cloth and gave them a good cleaning with some brake cleaner and some clean brake fluid. All is well 4 years later.