Although Adobe does offer platform swaps (e.g. Windows to Mac), it's only for current versions (currently CS6).
From CS5, your options are to crossgrade to CS6 (and pay the upgrade fee) or join the Creative Cloud (and pay the monthly fee).
Or you may prefer to investigate Photoshop Elements 11 for Mac.
Bear in mind that Adobe has introduced a one-version-back upgrade policy starting with CS6.
So, if you want to enjoy upgrade pricing in future, you must upgrade CS5 to CS6 in order to qualify for upgrade pricing to CS7 (whenever that ships).