Although Adobe offers platform swaps (Windows to Mac, Mac to Windows), they only apply to the current version (currently CS6).
Unless Adobe's policies have changed (and the article has not been updated), that article suggests that you'll have to purchase a "crossgrade" to Photoshop CS6 (Mac).
Or join the Adobe Creative Cloud.
Contact Adobe Customer Support and ask for a cross-grade (cross platform upgrade).
Also note that the Creative Cloud subscription allows you to download for both platforms, in any languages.