For those interested. I contacted Motu about the first version of MachFive with regards to MSW/Mac. It wasn't until MachFive2 that they sold a unified version. The MSW version/key of MachFive1 cannot be employed for the Mac (and vise versa).
Also, MachFive2 doesn't support OS9. So if you want MachFive on OS9 you have to find a specific Mac version of MachFive1.
MachFive2, and MachFive3 come in universal versions, and you can use the key on either platform (OS X, or MSW). Interestingly, both 2 and 3 support the PPC!