After install flashplugin-nonfree (aptitude install flashplugin-nonfree) we get an error saying that the file cannot be downloaded. Ok, the problem is that the server is using https not http. So all we have to do is edit the update-flashplugin-nonfree file located at /usr/sbin:

root@cicada_3301:~# nano /usr/sbin/update-flashplugin-nonfree

And change http for https at the line:

so the line must be:

Sorry for my English