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:
Nope. When you purchase on perpetual license, it's only good on that platform -- either Win or Mac, not both. On the Creative Cloud, you can install on both platforms.
If you want to run a perpetual licensed ver of CS6 on Mac, you either need to purchase another copy for Mac platform or contact Adobe Customer Support and ask them to cross-grade your Win version to Mac which means your Win version cannot be used anymore.