Go on http://maxwellito.github.io/chromecast-canal-plus and follow the instructions.
REQUIRED: This script only works in Chrome/Chromium with the Chromecast extension
JavaScript injection to cast videos from canalplus.fr to your Chromecast. The principle is simple: using a bookmark to inject injector.js on the current tab to find the video file links and display a little icon to cast them to your Chromecast.