I've just got an X100S. I've tried everything but I can't seem to get the overlay to display over the optical viewfinder.
The manual seems to suggest it should just always be on.
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You can turn the overlay on and off with the DISP/BACK button (page 21 of the manual).
You can customize what's shown in the overlay (pg. 75) with DISP.CUSTOM SETTING, and that includes the framing grid.
I should also caution you against wearing polarized sunglasses and shooting in portrait mode, which is the way I managed to inadvertently make the overlay disappear. ;)