First google hit on install .deb file
would have told you that
$ sudo dpkg -i install_flash_player_10_linux.deb
would probably do the job...
I think looking the package up in your repository would probably work too btw:
$ apt-cache search flash
adobe-flashplugin - Adobe Flash Player plugin version 10
If so, you can also
$ sudo apt-get install adobe-flashplugin
with the benefit of updates being pushed your way as part of the repo updates.