Unfortunately, the two are considered separate products. While you can use your PC license on up to two computers, they must be PCs.
Another alternative, however, is this:
If you have some kind of technology on your Mac that allows you to run Windows programs, such as (I believe) BootCamp, then I believe under that scenario you would be able to run your PC license on a Mac. However, this certainly isn't a formally approved way of using the game.
Barring that, however, you must purchase a Mac license separately.
Steve