If you have not done so already, be sure that you have the adobe repository setup (as described in my Repositories & Updates page), and SELinux has been configured properly (as described in my SELinux page). To install the plugin, simply type:
~>
sudo yum install flash-plugin
Hit Enter
and
press 'y'
when asked if you want to install the package.Restart Firefox to enable the flash plugin. You can go to any number of places, such as Adobe's Flash Player page, to test it. If the plugin is not working, you may have to logout of Gnome/KDE/Enlightenment first and log back in to restart the font server.