There is a generic Mach3 post in Fusion. Are you asking for a milling post or turning ?
If you look at the topic subject I suspect a turning one. So first try the generic fanuc. That should just work. If it doesn't try this one: http://cam.autodesk.com/posts/?p=tormach_slant-pro
Laurens Wijnschenk 3DTechDraw
AutoDesk CAM user & Post editor. Found out the hard way is the best way to win.