You cannot purchase the Mac version from IV. The Mac version was ported by the now-defunct Contraband Entertainment, and published by Ambrosia Software. You can buy the Mac version from their website
, but you will be purchasing version 1.31, and there is little chance that this will patched anytime soon -- as I said, the company that did the original port doesn't really exist any more, and ASW will not be doing any patches. If some brave soul, such as frenchfrog, decided to update the Mac port, it might happen, but it does not seem likely.
If you buy the boxed version from IV, you get the Windows version, and the Linux version. You can download that from IV. The Mac version is boxless, download only. Honestly, I would suggest that you get the Windows version, as it will still be supported in the future, and run it in Windows on your Mac via Boot Camp. It may also work under Parallels, but I am not as sure about that.
And, for those keeping score at home, this is my third helpful post. I'm batting over .200... that isn't right. :(