First congratz on your monitor, its a beauty.
-Go to overclocking menu on your monitor by using the joystick control on the back side
-Tick the overclock with the joystick ok (click) button (its a 5 way joystick, up, down, left, right and click)
-Choose the overclock refresh rate go up and down to make sure the slider moves, once you are on 100hz click again. (Most people dont click but just go back, this might be your problem)
-At this point the monitor will ask your confirmation, click ok again with the joystick.
-Monitor will restart itself and voila you have 100hz
Make sure you right click on windows and choose 100hz from Nvidia control panel after you do the above steps.
Let us know how it worked out for you.