This assumes you don't mind downloading to ~/Downloads and installing there. From terminal, run this:

sudo apt-get install adobe-flashplugin && cd ~/Downloads && wget http://fpdownload.macromedia.com/get/flashplayer/pdc/11.2.202.238/install_flash_player_11_linux.i386.tar.gz && tar zxvf install_flash_player_11_linux.i386.tar.gz && sudo cp libflashplayer.so /usr/lib/adobe-flashplugin/.

To see if we succeeded, try to watch this video: https://www.youtube.com/watch?v=oHg5SJYRHA0&hd=1