There is a program available called CloneKeys that runs on OSX. It is many years old but is basically just a key broadcaster to multiple windows of WoW running under OSX. Very limited by today's standards, but free.

I would advise against trying to multibox natively in OSX - in my experience, WoW under OSX is far less efficient with graphics routines etc. so that it would take a pretty hard core box to run 5 instances of WoW without much lag. You would be far better of booting into Windows under BootCamp and going the standard Windows MB methods.