Investor RT has a Mac specific version, but I haven't played around with it too much to be honest.
Java platforms are cross-platform, so you may have luck with that. For example, Oanda's FxTrade platform is java, so it works on mac, linux, windows, etc.
One thing you may consider is getting a virtualization program like parallels or VMWare. I use Parallels and it works out great! That way I can view Windows programs, and I'm not limited in my trading options because of it.
