As your brake pads wear out, the caliper adjusts itself so that you will have strong brakes throughout the life of the pads. If you look on the inside of the caliper you'll see a round piston coming out. This is what pushes on the brake pads from the back. Problem is, it's adjusted itself to match your worn out pads. Trying to get it over the new pads is like parking a Cadillac in New York City. You can do it, but the damage level will be high. Instead of destroying your new pads, you'll push the piston back to the starting point.
Take the c-clamp and place the end with the screw on it against the piston with the other end of the clamp around on the back of the caliper assembly. Now slowly tighten the clamp until the piston has moved far enough in that you can easily plop the caliper assembly over the new pads.