The extension utilize pidgin's dbus API, to integrate Pidgin into the GNOME shell. I only did some rough test with it only on my Fedora 16 machine, but I believe it should work ok everywhere else.
Copy the files over:
mkdir -p ~/.local/share/gnome-shell/extensions/pidgin-conversation-integration@kagesenshi.org
cp extension.js metadata.json ~/.local/share/gnome-shell/extensions/pidgin-conversation-integration@kagesenshi.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!