We are working on export to mac functionality in SynthEdit Version 1.2 (64-bit). What this means is you develop your plugin on the PC, which can then be exported to a Mac plugin. There is no Mac version of the SynthEdit editor (where you design the plugin).
To prepare for this you need to be developing with the 64-bit version of SynthEdit, because only this version than can export Mac plugins.
There are limitations with 64-bit export. Some of SynthEdit's graphical modules don't work. This is because 64-bit export uses a new graphics library called GMPI-GUI. Currently only basic knobs, sliders, list-entry, and image modules will display. This should be enough to build some plugins though.
The good news about 64-bit is any GUI you make will automatically be Mac-compatible.
Another limitation with 64-bit SynthEdit is that most 3rd-party modules (SEMs) are only available as 32-bit versions, which are incompatible. Initially you will be restricted to the modules that ship with SynthEdit. If you use 3rd party modules you will need the support of the developer to also make them available as Mac versions.
Initially the only format supported on Mac was VST3 64-bit.
[UPDATE] We now plan to support only Audio Unit (AU) Plugins on Mac.
I had a big setback with the graphics side of SynthEdit 64bit. I had to throw away some work and remake the Graphics API. I have 90% completed that now, so we can concentrate on stabilizing SE 1.2 . To speed things up a bit I am ignoring the Mac for now, so we can get stable Windows VST2 and VST3 export at least. Mac will be done after Windows is stable.