I found these solutions that should work:
the most common cause of 'Add to dictionary' being greyed out is because the document language and the Proffing language are not the same.
Go to File>Options>Proofing click on Custom Dictionaries, at the bottom of the dialogue box change Dictionary Language to All Languages.
See the last item (scroll down to bottom of page) on click here
"If, however, the Add to Dictionary command is unavailable (dimmed on the shortcut menu), this indicates that the language of the default dictionary differs from the language applied to the word you’re trying to add. By default, Custom.dic is set to All Languages; if you change it to, say, French, you will not be able to add an English word. This error might easily arise if you had created an additional custom dictionary for specific terms, set the language to something other than All Languages, set it as the default temporarily, and forgotten to reselect Custom.dic as the default.
Note: It has also been reported that sometimes the Custom.dic file inexplicably gets flagged as Read-only. The solution to that problem is to select the file in Windows Explorer, right-click, choose Properties, and clear the check box for "Read-only.""