New method - Adobe Flash Player

Add partner pepository :

sudo add-apt-repository "deb http://archive.canonical.com/ubuntu $(lsb_release -sc) partner"
sudo apt-get update

sudo apt install adobe-flashplugin

In your browser : about:plugins, and enable Flash Player

Restart your session

Ancienne méthode (before 2015) - Pepper Flash Plugin

Erreur : https://support.mozilla.org/fr/kb/plugin-adobe-flash-plante

Solution : http://askubuntu.com/a/449266

Chromium 34 in the main repos have started using Aura (early), which does not include support for NPAPI (this is a planned phaseout of NPAPI in Chromium). Therefore, you need to use Pepper Flash to be able to use Flash.

If you have Trusty, you can just run

sudo sudo apt-get install pepperflashplugin-nonfree.

If you don't have Trusty, you can use this PPA to install Pepper Flash for any supported Ubuntu version above Precise. Run the following commands to add the PPA and install Pepper Flash:

sudo apt-add-repository ppa:skunk/pepper-flash
sudo apt-get update
sudo apt-get install pepflashplugin-installer

Note that you need to configure Chromium to use Pepper Flash. To do this,

gksudo gedit /etc/chromium-browser/default

and add the following line to the end of the file on a new line:

. /usr/lib/pepflashplugin-installer/pepflashplayer.sh

Close all windows and re-open...