By default the Chromebook Flash plugin is not enabled. The Chromebook controls the Flash plugin on a per-website basis. The plugin is updated through the general ChromeBook OS update so it does not need to be updated manually.
- Visit the website that requires the Flash plugin
- Tap the green "Secure" link in the address bar
- Tap the down arrow next to the Flash plugin
- Tap the "Always allow on this site" option to enable Flash for this website
- Enjoy the Flash content!