The windows precompiled versions of the addons don't appear to be located in the main repository at the moment (now at https://github.com/OSGeo/grass-addons), but at http://wingrass.fsv.cvut.cz. The problem seems to be that the (incorrect) url is hardcoded in grass.
There is a ticket for this: https://trac.osgeo.org/grass/ticket/3455
Python scripts can be installed manually by copying the files as follows (example for v.transects.py):
v.transects.py to %APPDATA%\GRASS7\addons\scripts and link to \bin
v.transects.html to %APPDATA%\GRASS7\addons\docs\html\
image files to %APPDATA%\GRASS7\addons\docs\html\
Update %APPDATA%\GRASS7\addons\modules.xml to match the relevant section
You might need to update the name of the script in modules.xml to add the py extension (or rename the file).
You can find the addon in the modules table of the layer manager under addons.