Toadman is right on the location for the game saves.
However, if you're looking to move over save games from a PC... you can find those by viewing invisible files and folders and going into the "Application Data" folder inside your user folder.
Ex:
C:\Documents & Settings\Username\Application Data\SPORE\Games\
Additionally, you will want to copy over your creations from your My Documents folder. Be sure to do that before running SPORE on the Mac... otherwise SPORE won't load your creatures and creations with your saved game.