No worries mate, your not stupid (or even blind).
You can't see the byway on the map because it isn't shown
Highway authorities should inform the OS of any changes or additions to the definitive map so they can be added at the next reprint but it doesn't allways happen.
The maps for many areas are running behind by several years and if you already own a map of the area then it may have been several years old when you got it and that could be several years ago etc, etc.
All the other newish routes that I know of in Norfolk have eventually made it onto the OS maps so I'm sure this will too in time but don't hold your breath. It could well take several years before you can actually walk into Smiths and by a map with this lane shown and even then it will show the legal route which is TROed not the permissive alternative.
Don't worry about that though because they start and finnish at the same points and the county council have said that the permitted route will be signed so hopefully it will be apparent on the ground.
Any problems let me know and I'll do what I can to help out.
Cheers
Richard