Is the part shape just a circle like it appears in the pictures? If so, I'd measure the opening and just create a circle to match in Easel. You can enter the size by hand to get it precise. Select outside for the cut and make sure you pick the right endmill diameter when you start the carve. If that still doesn't work I suspect you might have some tuning to do on your machine.
Usual culprits are stepper motor power (ie: turn up X/Y axis pots), v-wheel tightness, slipping pulleys, and loose belts. There are lots of threads to give tuning advice.