If you have Windows installed on your Mac (via bootcamp) you can use the Windows version of MS Office. If you only have Mac OS, you need the Mac version. It's as simple as that.
I have microsoft Office 2007 which I have been using for my PC, but for uni I would like a mac. Would I be able to use MS office 07 on the mac without any problems or do I need to buy MS office for mac? (which will mean more money)