Robert Redl's Library tagged flashplugin →  View Popular, Search in Google

 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"
 Now "Close" and "Reload".

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