Hello @Pencil_Tester, and welcome to the forum!
@sooz, this is about Fizzyflower’s ULTIMATE MyPaint Brush Collection, not the FizzyFlower’s “ESSENTIAL” Brushset VERSION 2 
@Pencil_Tester, which of the three packages of MyPaint brushes did you try to install?
Was it the package “My Paint Bundles” (a ZIP-File with 18 BUNDLE-Files in it),
or the “MyPaint Krita Brush Presets(.kpp)” (another ZIP with 653 KPP-Files in several subfolders),
or did you try to install the " (Converted MyPaint brush engine 2 → 3) .MYB+.PNG brushes for programs with the mypaint brush engine (Gimp, Enve, etc)" (another ZIP with 292 MYB-Files + additional PNG’s) ?
Michelist