The extension utilize pidgin's dbus API, to integrate Pidgin into the GNOME shell.
It's easiest to link the subdirectory here to your extensions directory:
cd ~/.local/share/gnome-shell/extensions ln -sv <path to git>/pidgin-conversation-integration@katesenshi.org ./
restart Gnome shell (Alt+F2 type r and hit enter)
using gnome-tweak-tool, or https://extensions.gnome.org/local/ , enable Pidgin Conversation Integration extension
Enjoy!