I have two graphics card in my laptop.

However, I am unable to activate the NVIDIA one. I need to use it, because the integrated Intel card can't connect through DVI/HDMI.
I have the drivers installed, however I am unable to "select" the NVIDIA card.

How do I enable the second graphics card?

While running as suggested gives
sudo: nvidia-xconfig: command not found
Trying to install nvidia-xconfig fails as there's no such package.