Download: StuffItDeluxe12Windows.zip
Expand zip to a local file on your computer.
Double click to RUN: StuffIt12.0.0.17en.exe
When this is complete, 
 double click to RUN: 
StuffItPlugins.exe

When promted, enter serial number (you can copy and paste this):

WSDG-5LAKHCV-00011080

DO NOT REGISTER YOUR COPY. THIS HAS ALREADY BEEN DONE.
Bypass this.