I am going to go out on a limb and guess that there are going to be replaceable tops in case one is damaged, or you need to put a larger button top on for a specialty bit. I would go through your packaging and see if you have a random looking disc.
The solid button is an older design. New ones have a replaceable button, which is just a disc magnet. Like 11-ish mm in diameter. Meant to be replaceable because people sometimes forget to turn off their routers and then have to live with the shame of a damaged button forever. If it cannot be found in the box, just contact support.
I’ve found is that the magnet comes off every time I measure using one of the solid probes that came with the BitZero v2. I’m gonna have to find a way to stick it down.