I've never added zinc to beer yet, and I would only add zinc if I was doing like an imperial Czech pilsner or big Belgian strong, with all pale or pilsner malt and distilled water, because of the extreme stresses on the yeast and extreme relatively low amount of nutrients in the malt and water. For anything else, I think it's unnecessary. To add the zinc, I would take a standard zinc supplement tablet, break off about 1/16 of a piece and crush it and add to the boil. It wouldn't take much, as it's really a trace mineral -- the yeast probably only needs a few dozen ions of it in a 5 gallon batch, seriously. Nice thing about zinc supplements -- they'll cure the common cold a day or two faster than without, so they're handy to have around the house even if you don't use them for yeast nutrition. But I wouldn't recommend humans OR yeast eat them all the time. Only when necessary.