What version of View NX2 are you using? The latest version is 2.7.2, released 19th Feb. 2013. The version you have may predate the 50mm 1.8G, so distortion control wouldn't be available for it. That's no excuse for
everything being disabled, but...
If you'd prefer the English version (I don't know if French is your first language)
you can get it here.
EDIT: The following is an extract from View NX2 Help:
You cannot adjust the following RAW images:
- RAW images saved with the Capture NX software series.
- RAW images edited with nik color Efex Pro 2.0 using Nikon Capture 4 or PictureProject.
- RAW images shot or adjusted using an optional Picture Control that has not been installed in ViewNX 2.
For details, refer to "Picture Controls (RAW images only)" and "Supported Formats".
When you try to select and adjust multiple images that contain unavailable ones, only the available images will be adjusted.
About RAW images shot with the [Auto Distortion Control] function to [ON]
RAW images (NEF or NRW format) shot with the [Auto Distortion Control] function to [ON] will be displayed with the setting applied. ViewNX 2 cannot switch ON/OFF setting.
The ON/OFF setting of [Auto Distortion Control] function can be checked in [File & Camera Information] on the [Metadata] pane.
So, to get Auto Distortion Control for your 50mm 1.8G, you need to
update the D5100's Distortion Control Data firmware. That might just fix it all. Alternatively, you could turn off Auto Distortion Control in the camera.
Chris