Rated 1 out of 5 stars

I can download the dictionary, but there seems no way to install it.

This review is for a previous version of the add-on (3.1.8).  This user has a previous review of this add-on.

Heya, Dave,

Read the instructions here:
https://proofingtoolgui.org/en_installing.html

Basically, you can simply open the tools → add-ons → dictionaries, and drag'n'drop the .xpi file into this window.