If you've downloaded .deb package for Ubuntu on your desktop,maybe you can just double click on it and
gdebi will install it for you,if it's installed and I think it is,if it's not you can install gdebi with Synaptic,or you can do it the other way;open terminal,go to Desktop folder with command:
Code:
cd /home/you/Desktop
Change "you" with your name or whatever it should be there then run:
Code:
sudo dpkg -i flash-player package.deb
Change the name of the package to original name.Enter your password and hit <Enter>.That should do.
As already suggested,there's flash-player somewhere in the repositories,maybe you need to enable some of them in Synaptic package manager and search for flash-player then.