Nikon Fn button

[Dominantly]

I know exactly what you are talking about, and while I'm ashamed to admit it, I too couldn't get any kind of grid to appear with a button, but realized I had it enabled already and moved onto assigning the button to change my AF area mode.

I Just went back to see if I could get it to work, and it does.

So you can turn the grid on in the menu under subsection d2. If you set the function button (under f3) to Grid, how you turn it off and on is by holding the Fn button and then turning the main command dial (rear one). This will toggle it off and on.
I prefer it on at all times (why not, it helps you keep crap level), so I would use the button for something a bit more useful.

Hope this helps.

PS: It's also on page 197 of your manual. I keep this thing next to my keyboard because of this site.

 

[Derrel]

Eloquently stated. "read the fine manual". That's what RTFM stands for, right?

 

[Dominantly]

Frackin A' it does.