You can probably just copy the binary from the CD back to where you have mounted the harddisk containing your ubuntu installation. If you don't know where it is, you can probably find it at "places" on the left-top in the menu-bar
I do these things in a terminal, your filemanager may not have enough access-rights. Open a terminal at applications/accessories
To also restore the ownership and
sudo cp /sbin/dhclient3 /yourubuntumount/sbin/
to me, it seems easier if you do it the other way around, boot into your installation and copy the file from the CD, you end up copy-ing it like this.
sudo chown root:root /sbin/dhclient3
sudo chmod 755 /sbin/dhclient3
sudo cp /media/cdrom/sbin/dhclient3 /sbin/