You can use amule if you are trying to find very rare files. Note that the ED2K network is only fast when you have multiple file downloading at the same time and stay on the network for long periods of time (days and weeks). If you have enabled the software firewall included in OS X, you can open ports with System Preferences/Sharing/Firewall/New... You cannot use P2P effectively without opening ports. If you have routers, consult your router's manual on topics such as port forwarding/triggering and NAT.
If you have Azureus already, just google "gamename" (where "gamename" equals the public domain game you wish to acquire) _and_ the word "torrent." Download the torrent, which will open as an Azureus file on your Mac. That should be all you have to do.
Or you could go to download.com and search for the Mac version of what you want.