@liza
when you say she doesnt have enough memory, are you saying that she doesnt have enough hard drive or enough system RAM (Random Access Memory)? Remember both of them should not be confused with one another as they both have have completely different functions.
Even though Iḿ not a Mac User, I can say downloading is probably out of the picture. Its a game that needs 1gb of free Hard drive space to run and to leave no free space would probably cause some problems for OSX.
@Rachel
From my knowledge Unix (the system that Mac OSX is based on), I understand that any changes to the system generally require user administration. Did you or anyone in your house make any major changes (eg System software updates,installations or adding new hardware) in between the time when you last had the game running and when you discovered it wouldnt work? If any changes did occur, try reversing them and testing if the game will run. Also what is the error message that appears on your screen?
PS: I have the Windows version installed on my pc but am experiencing problems where the sound frequently pops or clicks or stutters, resulting in massive slowdowns. Is there anyone out there who has either the Mac or Windows version installed and suffers the same problem?