I need to reinstall Creative Suite 6 on a mac and it keeps asking for a old serial number, when given it the answer is it doesn't qualify for an up grade. How do I get past this, remember this is a reinstall.

Which version is the old serial number for?  Depending on when you acquired CS6 it could be either CS3+ or CS5+.

You might need to contact Adobe Support thru chat to get help with this.

For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
Make sure you are logged in to the Adobe site, have cookies enabled, clear your cookie cache.  If it fails to connect try using a different browser.

Serial number and activation chat support (non-CC)
http://helpx.adobe.com/x-productkb/global/service1.html ( http://adobe.ly/1aYjbSC )