It's quite easy:

1. Bring up the Camera Menu and under the CUSTOM SETTINGS MENU go to “f Controls”. Scroll down to “f4 - Assign Fn button”. Press the right arrow button once. You will now see a list of functions available. Select “Viewfinder virtual horizon”. Press the OK button.

2. Your function button is now set to show the virtual horizon tool in the viewfinder. To access it simply press the function button once. In the viewfinder you should now see two scales – one running left-to-right on the bottom of the screen, and on the right side of the screen you should see the same scale running from top-to-bottom. To turn off the virtual horizon press the function button once.

Source: https://www.nikonians.org/reviews/nikon-virtual-horizon-feature-two-great-ways-to-use-it