I have yet to come across a realtime trading application written specifically for Mac. Some firms, like Interactive Brokers, have Java applications that will run just fine on the Mac. Other than that, your best bet is to either use BootCamp to setup a dual boot option, or to run a Windows virtual machine using either VMWare Fusion or Parallels. Personally, I'm partial to Fusion just because I like being able to move the virtual machines back and forth between my Mac and Windows (PC) machines.
