How can I measure the wheel circumference?

The easiest way to measure the wheel circumference is to lay a string / chord around the tire.
Alternate method: Put a mark on the very bottom of the wheel (must be the one with magnet and sensor)  and ground. Push the vehicle forward until the mark on the wheel is the on the very bottom again, now put  a mark on the ground again. Finally measure between the two marks on the ground. This is the value you have to put in your acewell device.