To be able to generate nfo file for XBMC, first you have to choose the correct Media player.
So go to Options -> Theme Manager and select XBMC in the list :
In the “Create Files in” Option, select where the nfo file must be created, here it will be “C:\XBMC”
Click on the save button (the check).
Now on the Main Window, select a movie, TV show or music, Right click on a cover and
select “Create Cover”.
You will receive a confirmation box :
Now you can check that nfo was correctly created :