My understanding is that the book description is held on the Amazon server, not in the metadata. So you'll need Whispernet to be switched on to see the description. Also, there's no way of seeing it for non-Amazon books (as far as I know).
But that doesn't explain why your menu item is greyed-out. On my Kindle, if I try to access the description while Whispernet is off, it just prompts me to switch it on. As for non-Amazon books, the item isn't there at all.
As far as Calibre is concerned, that does let you edit the description. But, as far as I can see, the Kindle doesn't make use of that description.