Just a quick question: Why are you downloading the drivers from Nvidia directly? The packages available through
jockey (the Additional Drivers application) should be fine for most people and if you don't fit into "most people", using the X-SWAT PPA should give you the latest version of the nvidia driver.
Anyway, what you want is simple. Firstly you have to get out of X before you can install. You can do that by switching to a virtual terminal (tty) by pressing ctrl+alt+ F1 logging on and then running
sudo stop lightdm. Now run the installer. If it's still moaning about
sudoedit /etc/modprobe.d/nvidia-graphics-drivers.conf and fill it with this:
# This file was installed by nvidia-current-updates
# Do not edit this file manually
alias nvidia nvidia_current_updates
alias nouveau off
alias lbm-nouveau off
The driver will create that file when it's installed, but you just want to speed things along. You'll need to reboot and kill X again now.
Then run the installer and all should be fine.