I have a license for Photoshop CS6 which was installed on a PC that is now beyond repair. How can I install the software on my iMac? I still have the serial number that is registered to my Adobe Id?

Serial numnbers are platform specific so you will not be able to use it for the Mac if it was initially designated for a PC.  YOu will need to perform a platform swap.  See the following for information:

Change product platform or language
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You can swap languages/platforms for a latest version product if you follow the instructions at the following link:

http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platform-language-swap.html