I think I used the tip of my knife to gently scrape at the outside edge until I got they to buckle in and grabbed them with the needle nose pliers. They are copper and very soft, you will destroy them. But you have new ones, so no biggie.
Which Macs did you get? Staggered Slashes for me, definitely increased the sound by between 5-10 dB at idle, more when riding of course. I had a problem with the front pipe, the 2 pc bracket for the rear of the pipe was too large for the frame. I went to the hardware and bought some ribbon used to seal the doors on wood stoves. I wrapped that around the frame and put the bracket over, using it to take up the slack between the two. :mrgreen: