In the Booter utility, what does that radio button that specifies the kernel is in MacOS do?
It allows you to have a kernel in the MacOS HFS file system which the Booter will boot off of instead of booting off of a kernel file in the NetBSD FFS filesystem. This highly useful feature allows you to test kernels on your machine before actually going to the trouble of performing the entire NetBSD installation. If a kernel will actually work on your machine, then booting it from the MacOS will get you all the way through the boot process until it has to change the root device. So if you are interested in running NetBSD on your machine for the first time, this option is a good one to try.