Ian,
I just today purchased Photo Mechanic for my Mac. But the process should be the same.
If you purchase the "Media" version, Camera Bits will send you the program on a CD (or maybe a DVD). I did not opt for that choice.
Either way, you will get a serial number that, along with your name (optionally company name too), is used to activate Photo Mechanic. I entered this information in the trial version I was using and it became a full fledged registered product.
Scott