Warning, I'm assuming
you are using firefox.
I think you should be able to so it like this:
the file libflashplayer.so
which will be somewhere
on your system
As root, copy it to /usr/lib/firefox/plugins/
sudo cp /home/user/somewhere/libflashplayer.so /usr/lib/firefox/plugins/
should look like this:
root@vaio:~# ls -al /usr/lib/firefox/plugins/libflashplayer.so
-rwxr-xr-x 1 root root 8115888 2008-06-08 09:40 /usr/lib/firefox/plugins/libflas hplayer.so
If it doesn't then change its owner:group like this:
sudo chown root:root /usr/lib/firefox/plugins/libflashplayer.so
sudo chmod 755 /usr/lib/firefox/plugins/libflashplayer.so
Restart firefox, and everyone should be able to use flash.
It is best to install software as the root user, if you want everyone to be able to use it.