Unless I'm thinking of something else:
A) You don't set it in the Global Settings. You need to set it per game (aka Program Settings tab).
B) The option is in a dropdown box above all of those settings.
C) If this is a desktop machine and your monitor is connected to your GPU (you will have display ports from your motherboard at the top, and then display ports from your GPU down the bottom on the card itself), you will not getting this settings, as (almost) all programs will default run with your nVIDIA card.