Comment 191 for bug 173890

furicle, try this:
System -> administration -> software sources -> be sure than you have selected the "main", "universe", "restricted" and multiverse".
Then go to the Updates tab -> check "security", "updates" and "proposed"

after that, do this in the terminal:
sudo apt-get remove --purge flashplugin-nonfree
sudo apt-get install flashplugin-nonfree

$ sudo apt-get install flashplugin-nonfree
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  konqueror-nsplugins ttf-xfree86-nonfree xfs
The following NEW packages will be installed:
  flashplugin-nonfree
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B/18.1kB of archives.
After unpacking 160kB of additional disk space will be used.
Preconfiguring packages ...
Selecting previously deselected package flashplugin-nonfree.
(Reading database ... 145003 files and directories currently installed.)
Unpacking flashplugin-nonfree (from .../flashplugin-nonfree_9.0.48.0.2+really0ubuntu12.1_amd64.deb) ...
Setting up flashplugin-nonfree (9.0.48.0.2+really0ubuntu12.1) ...
Downloading...
--15:21:06-- http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_9_linux.tar.gz
           => `./install_flash_player_9_linux.tar.gz'
Resolving fpdownload.macromedia.com... 88.221.186.70
Connecting to fpdownload.macromedia.com|88.221.186.70|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 3,036,127 (2.9M) [application/x-gzip]

    0K .......... .......... .......... .......... .......... 1% 52.30 KB/s
   50K .......... .......... .......... .......... .......... 3% 28.06 KB/s
[...]
 2950K .......... .... 100% 32.34 KB/s

15:22:23 (39.04 KB/s) - `./install_flash_player_9_linux.tar.gz' saved [3036127/3036127]

Download done.
Flash Plugin installed.