Just to elaborate a bit:
Through CS6, Adobe Photoshop licenses are platform specific. A PC license, serial number or application itself is useless on a Mac.
If you have CS6, you can contact customer service and request a platform swap, also called a cross-grade.
As J.J. Mack says, if you are on a monthly subscription, you just download the Mac version.
If you are on CS5 or earlier, you are out of the loop unless you upgrade or buy a new license.