Stylus buttons do not work in Krita's canvas. See:

https://bugs.kde.org/show_bug.cgi?id=342105

Guys there say it's a problem in HUION's end. Still, it would be nice actually having that circle of brushes and color picker available.

I tried Autohotkey but it doesn't seem to work (please send your working scripts if you have them.)

Quick Edit: By the way, I already know you can make a shortcut to open the pop-up palette and set a button on the tablet itself. I want to be able to open it by clicking the stylus button.

Same issue on my Viao. Right now I click on the tool bar then go back and right click on the canvas. It registers the first right click on the canvas when you click off somewhere else. Not the most elegant solution.

I set an alternate key to activate the popup. Preferences->Canvas Input Settings -> Show Popup Palette. There I added in another shortcut (mine was "Z").