In order to get Quod Libet working with your Libre.FM account you need to get the latest version of the qlscrobbler.py plugin and put it in your home plugins folder. You can get the last version of the plugin here: http://code.google.com/p/quodlibet/source/browse/#hg/plugins/events .
mkdir -p ~/.quodlibet/plugins/events cd ~/.quodlibet/plugins/events wget http://quodlibet.googlecode.com/hg/plugins/events/qlscrobbler.py
Once downloaded, you need to activate the plugin, start Quod Libet and go in the Music/Plugins Menu, tick Audioscrobbler Submission and configure the right panel, selecting Libre.FM and filling your username and password.
Notes: If Quod Libet was already running, you might need to click on refresh in the plugins menu in order to see the latest version of the Audioscrobbler Submission plugin. If you have a problem, please double check you username and password and make sure there are no warnings for this plugin when you click on the Show Errors button of the plugins panel.