zBrush (OS X) seems to crash (it hangs) when I try and create an Alpha out of a mask. It is a 493,000 poly tool with 3 subdivision layers and so it doesn’t strike me as being too large.
I am a noob to Brush and so I might be missing a tick here.
Any suggestions?
Does your model have UV mapping? The alpha is created to fit the UV map. If there’s no UV map… well, it doesn’t work so good. 
I am having the same problem but I went to sub division level 1 and Enable UVs is gray. I clicked Disable and then Enable to make sure. I went to click create alpha and it crashes. I tried it on sub D levels 7 6 and 5. All crash. How can I create an alpha to use it as a stencil? I thought I could click create alpha and then in the alpha menu select stencil.
aberrant, just enabling UV’s does not assign them so it could still lead to a crash. you need to map it, try AUV or GUV