Chromium no longer comes with flash installed. But there is a fix.
First thing you can do is simply start Linux Mint Software Manager and get the Pepper Flash Player – browser plugin. The package name is pepperflashplugin-nonfree.
You might be able to just click this link:
apt://pepperflashplugin-nonfree
If you need more help, please see link below:
https://wiki.ubuntu.com/Chromium/Getting-Flash
