Your CS6 Windows serial number is operating system specific: it only works on Windows.
Downloading the Mac version from Download and Installation Help and entering your Windows serial number will not work.
You have a few options:
- Install Windows (Bootcamp, Parallels etc) on your Mac and install CS6 in the Windows virtual machine on your Mac
- Do a platform swap to switch your Windows version to a Mac version
Order product | Platform, language swap - Buy CS6 for Mac (then you'll have a Windows serial and a Mac serial)
- Join the Cloud. The CC version can be installed on Windows and/or Mac.