Where are WEKA installed packages stored
I don't know how WEKA stores the extra packages on other installations, but for my Windows 10 with WEKA 3.8 it was here:

WEKA stores the extra packages in a direcory called packages. This package was in my user directory and under the wekafiles folder. 

Example: "C:\Users\myUser\wekafiles\packages....."

Hope this helps someone else.