Download the 64-bit Linux version of the plugin. The version I used was here, in tar.gz format.
Next, extract the file into your home directory. It should extract one file, libflashplayer.so.
Finally, open a terminal, and type the following:
sudo mkdir /opt/google/chrome/plugins
sudo mv ~/libflashplayer.so /opt/google/chrome/plugins/
Restart Chrome, and type
into the address bar. You should see the flash player plugin you just installed!