Just installed Zbrush 2018 with Sculptris Pro and was watching some of the video tutorials on using Sculptris Pro. Being a noobie, I can’t figure out why The SP tool is greyed out. I’ve created a sphere3D and am in edit mode. Also on a Mac.
Here’s screenshot of my Zbrush canvas with Activate Sculptris Pro mode not accessible.
Which step have I missed for turning on Scultptris Pro? Thanks.
If the Sphere3D is still a primitive(basically a shape define purely through a mathmematical formula you can change using the Tool: Initialization sliders) then you will likely have to turn it into a Polymesh3d before being able to do any sculpting operations on it.
Use Tool: MakePolymesh3D (found at the top of the tool menu) and see if that will let you use Sculptris Pro.
OK, apart from making sure that your mesh is a polymesh object that has no subdivisions and that you are in edit mode, select one of the painting tools like the standard or clay brush. Even if the button is greyed out, try pressing it anyway when you have the brush selected. I’ve had something similar happen to me.
Cheers,
Paul 
(mix_mash)