Well, since it wasn't anything "normal", I wiped the install (uninstall and deleted the Mach3 folder after saving my mill file). I installed the stable release with zero options, and it worked. So, somehow, the original install was screwed up. I had trying just re-installing, which didn't fix anything.
Oh well, it seems to work now. I'll try upgrading to the development version, but I bet that is going to work now too.