Yes, theres no way to play in native OSX.
youll need:
An Intel Mac
A copy of Windows XP/ or Vista.
Open spotlight ( the magnifiying glass in the upper right corner) in OSX, and type Boot Camp, and you should see the Boot Camp Assistant (im also assuming you have leopard, I think Tiger needs a seperate download), which will walk you through the process of installing windows. Once windows is installed, you boot to that, then you download the aRO installer and install and play.
If your Mac runs off of a PowerPC processor (older macs), or you don't have access to a copy of windows, unfortunately you're out of luck.