That's easy! If you haven't registered in the member area, go do that, and you can download the Mac OSX version, and you're done!. You can find your serial number under Help > ArtRage Serial Number in the program.
Steps:
Go to
https://members.artrage.com
1/ Click 'create account' and follow the instructions (if you do not currently have an account).
2/ Log in, then click 'register' and enter your serial number.
3/ Click 'save' and refresh the page.
You should now see your serial number listed on the page and you can click 'download' to download any of the latest versions (Windows or OS X).