See the instructions for how to Install an Isotammi addon using the Gramps’ Addon Management feature.
For manual installation, the downloaded archive just has to be decompressed to go in its own folder in the Gramps User Directory plugins folder.
For Windows, this is located at
%AppData%/gramps/gramps51/plugins/
The next time you restart Gramps, the plug-in should register and be available in whatever portion of the GUI that the programmer set. (It could be a tool, view mode, Gramplet report… etc ad nauseum.)