Hey, I downloaded 2 plugins and set put them in ZStartup\ZPlugs64 for some reason they are not in the list in ZBrush. I have 2 other plugins that are showing up just fine.
There are 2 of those folders. Some scripts/macros/plugs go in 1, and some in the other. I usually just drop in both or test and switch them. I'll post the exact path when I'm at the computer shortly.